Chocolate Dreams: Is chocolate good for your heart? The evidence isn't very impressive:

Eleven people were given 46 grams (1.6 ounces) of dark, flavonoid-rich chocolate every day for two weeks, while 10 others received dark chocolate with low-flavonoid content.

At the end of the trial, the researchers used ultrasound to measure how well blood vessels are able to relax if blood flow increases - called flow-mediated dilation.

They examined the brachial artery in the arm. How well this artery dilates indicates how coronary arteries are behaving.

In the group eating flavonoid-rich chocolate, blood vessel dilation increased by a tenth, while it fell by as much in the other group.

I love chocolate as much as the next person (probably more), but this seems pretty weak. It's a very small study showing very small changes in the caliber of a large vessel in the arm. Any conclusion that it helps the heart is purely speculative.