A recent study in the British Journal of Nutrition  has found that dark chocolate contains antioxidants that are protective to DNA, but this effect only lasts for a day. Researchers in Milan, Italy, measured plasma epicatechin levels, DNA damage in mononuclear blood cells, and plasma total antioxidant activity in 20 volunteers on a balanced diet with standardised levels of antioxidants. After a washout period the subjects were given 45g of either dark chocolate (DC, containing 860 mg polyphenols, of which 58 mg epicatechin) or white chocolate (WC, no epicatechin).
The results found that increased levels of epicatechin in the blood of those who had eaten the dark chocolate lasted for nearly a day; between 2 hours and 22 hours after DC intake. This corresponded with lower levels of DNA damage to the blood cells, but eating the dark chocolate did not affect total antioxidant activity. Eating WC did not make any difference to the factors measured. The researchers conclude: "DC may transiently improve DNA resistance to oxidative stress." They add: “the present results are clinically encouraging especially in the field of the diet therapy of obesity, pathology related to greater incidence of cardiovascular disease and cancer”. Unfortunately regular consumption of dark chocolate does not increase long-term epicatechin levels, so according to this study, dark chocolate must be consumed daily to get these benefits.
Chocolate has long been known to be an important part of a healthy diet According to Donatella Lippi of the Department of Anatomy, Histology and Legal Medicine, University of Florence, Italy :
The Aztecs believed that cocoa pods symbolized life and fertility, and that eating the fruit of the cocoa tree allowed them to acquire wisdom and power. Cocoa was said to have nourishing, fortifying, and aphrodisiac qualities.
One well-researched benefit of chocolate is improving the health of the heart. In a Data from The Stockholm Heart Epidemiology Study showed how of 1169 non-diabetic patients having their first heart attack, those who were regular consumers of chocolate were more likely to survive.  A study on dark chocolate published in the International Journal of Cardiology  measured the effect of 45g of dark chocolate on blood circulation in the coronary arteries as measured by doppler ultrasound. After two weeks of daily intake the researchers conclude:
Flavonoid-rich dark chocolate intake significantly improved coronary circulation in healthy adults, independent of changes in oxidative stress parameters, blood pressure and lipid profile, whereas non-flavonoid white chocolate had no such effects.
Epicatechins are a type of polyphenol antioxidant in the catechin family. Catechins are found in tea, wine, fruits and vegetables as well as dark chocolate. However it is the bitter principles in the chocolate that contain the beneficial antioxidants: An editorial in The Lancet  points out that some chocolate manufacturers may darken the natural cocoa solids and remove the bitter flavanols, "so even a dark-looking chocolate can have no flavanol". In addition, cacao colouring can contain more than the maximum EU permitted level of mercury (1 mcg/g).  Manufacturers rarely label their products with this information. In addition, 45g of dark chocolate contains about 200 calories, so calorific intake must be taken into account as part of the risk/benefit calculation. One way round that might be to use raw cacao nibs which contain no sugar and are also unheated, thereby likely to have a higher catechin content, although the consumer may not see the benefit of this over drinking red wine, for example.
Therefore the amount of chocolate consumed is not necessarily proportionate to it's health benefits: Another paper from the British Journal of Nutrition  demonstrated that even doubling the the polyphenol content in the same size dose of chocolate had no significant dose-related benefits:
It was observed that the 500 mg polyphenol dose was equally effective in reducing fasting blood glucose levels, systolic BP and diastolic BP as the 1000 mg polyphenol dose suggesting that a saturation effect might occur with increasing dose of polyphenols.
Dark chocolate is suitable for individuals of all blood groups.  However, a recent paper in the Journal of Cardiovascular Pharmacology  suggests that in addition to the well-known antioxidant effects, one way chocolate may directly help cardiovascular system is by improving nitric oxide function. Nitric oxide recycling is an important function that can sometimes be inadequate in those with the B antigen (blood groups B and AB), probably due to genetic linkage of the argininosuccinate synthase enzyme. 
When giving the gift of chocolate to loved ones it may be prudent to ensure adequate polyphenol content, absence of colouring, and to draw attention to the health benefits of both moderation and regular consumption.
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