Health Benefits

Researchers found eating cocoa-rich foods was associated with an average 4.7-point lower systolic blood pressure (the top number in a blood pressure reading) and 2.8-point lower diastolic blood pressure (the bottom number). Researchers say those effects are similar to those found using one-drug therapy with common blood pressure-lowering medications such as beta-blockers and ACE inhibitors. Applied to the population as a whole, researchers estimate that the blood pressure-lowering effect associated with cocoa would be expected to reduce the risk of stroke by about 20%, coronary heart disease by 10%, and death from all causes by 8%.
Source: WebMD

Dark chocolate is very high in flavonoids. These help lower LDL (bad) cholesterol, and improve blood flow by relaxing the walls of blood vessels, as well as reducing the aggregation of platelets. Flavonoids also have been proven to have the same blood thinning properties similar to those of aspirin. Additionally, scientists have found that the flavonoids in chocolate can help prevent skin cancer by making the skin denser and thicker.
Source: American Society of Nutrition, Journal of Nutrition, 2006 + The vasculoprotective effects of flavonoid-rich cocoa and chocolate. Nutrition Research, Volume 24, Issue 9, Pages 695-706, M. Engler

Dark chocolate has twice the number of antioxidants as red wine and three times as many as white tea. These help soak up those free radicals that cause cancer, heart disease and aging.
Source: M Karim, K McCormick, CT Kappagoda - Journal of Nutrition, 806

Cocoa is believed to be useful in fighting diabetes as it makes the body more responsive to insulin which guards against type 2 diabetes and other cardiovascular problems.
Source: The American Society for Nutritional Sciences J. Nutr. 133:3149-3152, October 2003

For one’s head, chocolate has several compounds, such as anandamine (similar to THC), which helps reduce depression and increase a sense of wellbeing, and phenylethylamine, which increases attention span, and is also the same chemical released when one falls in love. Chocolate also contains tryptophan, a building block of serotonin, which is a ralaxation-inducing neurotransmitter.
Source: Mood state effects of chocolate: Journal of Affective Disorders, 2006

For your teeth, chocolate has fat that is believed to protect the teeth against tooth decay and that the tannins in chocolate inhibit the formation of plaque. Cocoa has also been proven to be a more effective than fluoride in fighting cavities as the cocoa powder seems to harden tooth enamel.
Source: www.medicalnewstoday.com/articles/71158.php