Thanks to ChiroAccess for access to these materials!
The evidence supporting the antihypertensive effects of cocoa has been building over the last few years, and a January 2010 systematic review and meta-analysis of 10 randomized controlled trials, comprising 297 individuals confirms the BP-lowering capacity of flavanol-rich cocoa products.  The high level of flavanols found in the cocoa plant is believed to be responsible for the lowering of blood pressure.
Another study, published this month (March 2010) compared a lower (6g/day) with higher dose (25g/day) of dark chocolate, and the results suggest that, although chocolate may lower blood pressure, there was no added benefit to taking higher doses. The 25g per day group experienced slight increases in body weight with no additional improvements in BP. 
Read the rest of this fascinating review at ChiroAccess
1. Effect of cocoa products on blood pressure: systematic review and meta-analysis
American Journal of Hypertension 2010 (Jan); 23 (1): 97-103
2. Low vs. Higher-Dose Dark Chocolate and Blood Pressure in Cardiovascular High-Risk Patients
American Journal of Hypertension 2010 (March 4) [Epub ahead of print]
You may also want to refer to our Bioflavonoids Page for more information on the benefits of flavanols.
I would be remiss by not mentioning that chiropractic care also has a long history of reducing blood pressure, while having no impact on your waistline. You may find our page on Chiropractic and Blood Pressure Page to be of interest.