Participants of the study were 44 adults with either upper-range prehypertension or stage 1 hypertension (see chart) . After 18 weeks, those who were consuming 6.3 grams (about 1 square inch) of dark chocolate daily reduced their systolic BP by an average of 2.9 mm Hg, their diastolic by 1.9 mm Hg. (This is about the reduction seen in participants of the HOPE Study who were on an ACE inhibitor.)
The chart is extracted from the NIH's brochure: The Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure.
If you'd like to give this a try, you may want to eat the small amount of chocolate every day. In this study, there was no BP-lowering effect for the first 6 weeks. A gradually increasing effect became apparent after that, with the strongest effect at the end of the study (18 weeks).
1 Effects of Low Habitual Cocoa Intake on Blood Pressure and Bioactive Nitric Oxide