From this article Small Doses Of Chocolate Could Help Heart these quotes:
They found people who had an average of six grams of chocolate per day --- about one square of a chocolate bar -- had a 39 percent lower risk of either a heart attack or stroke.
Experts think the flavonols contained in chocolate are responsible. Flavonols help the muscles in blood vessels widen, which leads to a drop in blood pressure.
"It's a bit too early to come up with recommendations that people should eat more chocolate, but if people replace sugar or high-fat snacks with a little piece of dark chocolate, that might help,"
"However, before you rush to add dark chocolate to your diet, be aware that 100 grams of dark chocolate contains roughly 500 calories. Eating too much chocolate will have negative effects that far outweigh the positive effects of cocoa."