In my quest to the right chocolate dose recently discovered a letter in the BMJ about this subject. The authors did a meta analysis. Their results suggest that dark chocolate is superior to placebo in reducing blood pressures of more than 140 mm Hg systolic or more than 80 mm Hg diastolic.
Daily flavanol dosages ranged from 30 mg to 1 g (equivalent to 6.3 g to 100 g of milk or dark chocolate), and interventions ran for two to 18 weeks. Data were insufficient to provide confident answers on optimal dosage and time frame.
However, they question the practicability as a long term treatment. Can you imagine that? They found that 50 g daily of chocolate was significantly less acceptable to participants as a long term treatment for high blood pressure than one capsule daily of placebo or tomato extract (73% v 100%).