Blueberries show significant health benefits. A number of researches confirm these benefits and continuously provide new findings. The following data might be impressive and promising, however keep in mind that for a good health it is important to maintain a balanced diet and exercise.
• Only 57 calories of low fat per 100g. Blueberries are very low in Saturated Fat, Cholesterol and Sodium. They are also a good source of Dietary Fiber, and a very good source of Vitamin C, Vitamin K and Manganese.
• Blueberries are near the top when it comes to antioxidant activity per serving. Antioxidants work to neutralize free radicals — unstable molecules linked to the development of cancer, cardiovascular disease and other age-related conditions such as Alzheimer’s. Substances in blueberries called polyphenols, specifically the anthocyanins that give the fruit its blue hue, are the major contributors to antioxidant antioxidant activity.
• If you want to maximize your antioxidant benefits from blueberries, go organic! A recent study has directly compared the total antioxidant capacity of organically grown versus non-organically grown highbush blueberries (Vaccinium corymbosum L., var. Bluecrop) and found some very impressive results for the organically grown berries. Organically grown blueberries turned out to have significantly higher concentrations of total phenol antioxidants and total anthocyanin antioxidants than conventionally grown blueberries, as well as significantly higher total antioxidant capacity. Numerous specific antioxidant anthocyanins were measured in the study, including delphinidins, malvidins, and petunidins. The antioxidant flavonoid quercetin was also measured.
• Reduction of old age problems also aided by the antioxidants in blueberries.
• Visual retention is increased by a substance known as anthocyanoside found in the fruit. This element prevents early loss of sight.
• Aids in the prevention of infection of the urinary tract in humans
• Anthocyanins present in large quantities in blueberries assist in the eradication of E. coli.
• Pterostilbene aids in the reduction and maintenance of cholesterol levels in blood.
• These fruits are also associated with diseases such as dementia and Alzheimer disease. It has been proven that the consumption of this fruit aids in the protection of the brain and prevention of undue stress.
• Minimization of the threat of ovarian cancer in females is aided by the presence of Kaempferol in blueberries.
• Diarrhea and also issues associated with constipation can be controlled by the consumption of this fruit.
• Presence of the substance known as tannins in blueberries assists in the reduction of the inflammation of the digestive tract.
Blueberries are literally bursting with nutrients and flavor, yet very low in calories. Recently, researchers at Tufts University in Boston, Massachusetts analyzed 40 fruits and vegetables for their antioxidant capability. Blueberries came out on top, rating highest in their capacity to destroy free radicals.
Antioxidants help neutralize harmful by-products called “free radicals” that can lead to cancer and other age-related diseases. Anthocyanin - the pigment that makes blueberries blue - is thought to be responsible for this major health benefit.
An Antioxidant Powerhouse
Packed with antioxidant phytonutrients called anthocyanidins, blueberries neutralize free radical damage to the collagen matrix of cells and tissues that can lead to cataracts, glaucoma, varicose veins, hemorrhoids, peptic ulcers, heart disease and cancer. Anthocyanins, the blue-red pigments found in blueberries, improve the integrity of support structures in the veins and entire vascular system. Anthocyanins have been shown to enhance the effects of vitamin C, improve capillary integrity, and stabilize the collagen matrix (the ground substance of all body tissues). They work their protective magic by preventing free-radical damage, inhibiting enzymes from cleaving the collagen matrix, and directly cross-linking with collagen fibers to form a more stable collagen matrix.
In studies at the USDA labs at the Arkansas Children’s Nutrition Center, Little Rock, AR, blueberries ranked among the highest in antioxidant activity when compared to more than 100 other foods. , 
In another study at Tufts University, neuroscientists discovered that feeding blueberries to laboratory rats slowed age-related loss in their mental capacity, a finding that has important implications for humans. Again, the high antioxidant activity of blueberries probably played a role. (Bickford, P.C. et al. Society for Neuroscience Abs. 1998, 24:2157)
At a symposium about the health benefits of berries, reports had shown that the consumption of blueberries could lower the risk against Alzheimer's disease and other age related issues. In the same symposium studies on inflammatory mediators in glial cells of the rat brain showed that blueberry extracts could attenuate braininflammation. 
Subsequent studies on mice with Alzheimer’s-like symptoms demonstrated that blueberry supplementation caused signal transmission between brain cells to approach normal activity.(Lau, F.C. et al. Neurobiol. Aging 2005, 26S:128-132.
Ongoing research points to a possible relationship between blueberries and healthy brain function. In a study at the USDA Human Nutrition Research Center in Boston, a diet rich in blueberries reversed some loss of balance and coordination and improved short-term memory in aging rats (Joseph, J.A. et al. J Neurosci. 1999, 19:8114-21.)
Blueberries have been found to assist with prevention of age-related neurodegenerative diseases. One study concluded that regular consumption of blueberries and strawberries could “exert protection against age-related deficits in cognitive and motor function.”
A recent study found wild blueberry juice enhanced memory and learning function in older adults while reducing blood sugar and decreasing symptoms of depression. (Krikorian, R., Shidler, MD., Nash, TA., et al. epartment of Psychiatry, University of Cincinnati Academic Health Center, Cincinnati, Ohio, USA. Journal of Agricultural and Food Chemistry 2010 Apr 14;58(7):3996-4000.)
Proanthocyandin (a polyphenol similar to the beneficial chemicals found in grapes and wine) found in blueberry leaves has shown a strong effect in blocking the replication of the Hepatitis C virus. While proanthocyandin can be harmful its effective concentration against HCV was 100 times less than the toxic threshold, and similar chemicals are found in many edible plants, suggesting it should be safe as a dietary supplement. The research was made with Rabbit - Eye (a variety for southern areas).
Preliminary studies at Rutgers University in New Jersey suggest that antioxidants called proanthocyanidins, which are found in blueberries, can inhibit infection-causing bacteria from adhering to the lining of the urinary tract (Schmidt, B. M. et al. J Agric Food Chem. 2004, 52:6433-42.). Another research has concluded to the same result.
Another research suggests blueberries can help prevent urinary tract infections. Because urinary tract infections (UTIs) are caused by bacteria in the stool, dietary factors may affect the risk of contracting a UTI by altering the properties of the fecal bacterial flora.
While wine, particularly red wine, is touted as cardioprotective since it is a good source of antioxidant anthocyanins, a recent study found that blueberries deliver 38% more of these free radical fighters. In this study, published in the Journal of Agriculture and Food Chemistry, researchers found that a moderate drink (about 4 ounces or 120ml) of white wine contained 0.47 mmol of free radical absorbing antioxidants, red wine provided 2.04 mmol, and a wine made from highbush blueberries delivered 2.42 mmol of these protective plant compounds.
Another study found regular consumption of blueberries (one cup per week) can lower blood pressure and perhaps speed up metabolism, due mostly to the high amount of anthocyanins (antioxidant flavonoids that also create pigments found in plants) found in blueberries .
Extracts of bilberry (a cousin of blueberry) have been shown in numerous studies to improve nighttime visual acuity and promote quicker adjustment to darkness and faster restoration of visual acuity after exposure to glare. This research was conducted to evaluate claims of bilberry's beneficial effects on night vision made by British Air Force pilots during World War II who regularly consumed bilberry preserves before their night missions.
Laboratory studies published in the Journal of Agricultural and Food Chemistry show that phenolic compounds in blueberries can inhibit colon cancer cell proliferation and induce apoptosis (programmed cell death).
Extracts were made of the blueberry phenols, which were freeze-dried and further separated into phenolic acids, tannins, flavonols, and anthocyanins. Then the dried extracts and fractions were added to cell cultures containing two colon cancer cell lines, HT-29 and Caco-2.
In concentrations normally found in laboratory animal plasma after eating blueberries, anthyocyanin fractions increased DNA fragmentation (a sign that apoptosis or cell death had been triggered) by 2-7 times. Flavonol and tannin fractions cut cell proliferation in half at concentrations of 70-100 and 50-100 microg/mL, while the phenolic fraction was also effective, but less potent, reducing proliferation by half at concentrations of 1000 microg/mL. Bottomline: eating blueberries may reduce colon cancer risk.
Also for colon cancer: http://njaes-clone.rutgers.edu/pubs/blueberrybulletin/2007/bb-v23n23.pdf
Among their rich supply of phytonutrients, blueberries include a flavonoid called kaempferol. Research calculating flavonoid intake in 66,940 women enrolled in the Nurses Health Study between 1984 and 2002 revealed that women whose diets provided the most kaempferol had a 40% reduction in risk of ovarian cancer, compared to women eating the least kaempferol-rich foods. In addition to blueberries, foods richest in kaempferol include tea (nonherbal), onions, curly kale, leeks, spinach, and broccoli.
A significant 34% reduction in ovarian cancer risk was also seen in women with the highest intake of the flavone luteolin (found in citrus). Int J Cancer. 2007 Apr 30; Am J Clin Nutr. 2004 May;79(5):727-47.
Blueberry phytochemicals inhibit growth and metastatic potential of MDA-MB-231 breast cancer cells through modulation of the phosphatidylinositol 3-kinase pathway. 
Blueberries can help relieve both diarrhea and constipation. In addition to soluble and insoluble fiber, blueberries also contain tannins, which act as astringents in the digestive system to reduce inflammation.
Reportedly, dried blueberries have a long history of use in Sweden as a treatment for diarrhea. Dr. Varro Tyler in his book, Herbs of Choice, recommends either chewing dried blueberries or making a tea by boiling crushed dried blueberries for about 10 minutes.
The helpfulness of blueberries for diarrhea appears to be due to the fact that they contain tannins, which act as an astringent, contracting tissue and reducing inflammation and secretion of liquids and mucus. Blueberries also contain substances called anthocyanosides, which have antibacterial properties, as well as being a good source of antioxidants. Lastly, blueberries are another source of the soluble fiber pectin. 
Blueberries contain the same compounds found in cranberries that help prevent or eliminate urinary tract infections . The source is a commercial product.
In addition to their powerful anthocyanins, blueberries contain another antioxidant compound called ellagic acid, which blocks metabolic pathways that can lead to cancer. In a study of over 1,200 elderly people, those who ate the most strawberries (another berry that contains ellagic acid) were three times less likely to develop cancer than those who ate few or no strawberries. In addition to containing ellagic acid, blueberries are high in the soluble fiber pectin, which has been shown to lower cholesterol and to prevent bile acid from being transformed into a potentially cancer-causing form.