Several studies have shown that the consumption of cocoa flavanols can improve vascular function, blood-pressure, and raise levels of HDL ("good") cholesterol. Positive results have been seen with 50 mg to 200 mg of flavanols per day, and, in Europe, products with at least 200 mg may legally claim they "promote normal blood flow" (the U.S. has not yet permitted such a claim).
The flavanol content of cocoa and chocolate products can vary tremendously and most do not list their flavanol levels on their labels. But ConsumerLab.com tested a wide variety of cocoa, cacao, and chocolate products and you can find their flavanol levels (as well as amounts of contamination with the toxic metals cadmium and lead) in the Review of Cocoa Powders, Extracts, Nibs, Supplements and Chocolate >>
Also see these related CL Answers:
Why does dark chocolate have iron in it? Is it, or cocoa, a good dietary source of iron? >>
Which dark chocolate bar has the most flavanols with the least calories? >>
Why is there so much cadmium, a toxin, in cocoa powders but not in dark chocolate? >>
How much fat is there in chocolate? Is it saturated fat? >>
How much caffeine is really in dark chocolate bars? >>