Posting this because there is more detail in this article than others--
http://www.sciencenews.org/articles/20050813/food.asp Ferri and his colleagues tested a diet containing 100 grams per day of dark chocolate, in the form of commercially available "Ritter Sport" bars sold by Germany-based company Alfred Ritter. Blumberg's lab determined that each bar contained about 100 milligrams of flavanols, which are flavonoids that are relatively easy to measure. Blumberg estimates that there were about 400 milligrams of other flavonoids in each bar.
For 15 days, 10 volunteers with high blood pressure got those bars and 10 others ate white chocolate bars that contained no flavanols. After a 1-week break, the two groups switched the chocolate they were eating.
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During the half month when volunteers ate dark chocolate, their average systolic blood pressure decreased from 136 to 124 millimeters of mercury. Diastolic blood pressure fell from 88 to 80. There was no change in blood pressure in people while they ate white chocolate bars.
Furthermore, dark chocolate consumption accelerated the body's metabolism of blood sugar, or glucose, a process that involves the hormone insulin. Impaired insulin function can lead to diabetes. Dark chocolate also lowered cholesterol in the hypertensive patients, the researchers report in the August Hypertension.