A recent very detailed and clean study on the effects of a small serving of dark chocolate on blood pressure was just published in the Journal of the American Medical Association.
I was going to do a little Blogging on this interesting study but I read the Fanatic Cook's wonderfully insightful comments. I'd direct you to her discussion, instead: A small daily dose of dark chocalate lowers blood pressure at http://fanaticcook.blogspot.com/. I couldn't have said it any better.
By the way, the authors of the study had no financial ties to the chocolate or cocoa industry. Refreshing.