The healthy effects come from chemicals called flavonoids that are found in cocoa powder, according to researchers at Tufts University, Boston.
The study focused on 20 people with hypertension, half of whom ate a bar of dark chocolate every day for two weeks.
The other half ate white chocolate, which does not contain flavonoids.
Those who ate the dark chocolate had a drop in blood pressure, an increase in blood flow, and were more sensitive to insulin, a hormone that regulates sugar in the body.
While previous studies have suggested that flavonoid-rich foods such as fruit, tea and red wine could be good for the heart, this is one of the first clinical trials to look at dark chocolate’s effect on blood pressure.
“It’s not saying that chocolate is now a health food,” warned Jeffrey Blumberg, who led the study.
“It suggests that cocoa flavonoids appear to have benefits on vascular function and glucose sensitivity.”
However, Mr Blumberg said the statistics were “significant” and represented the kind of reduction in blood pressure often linked to a healthy diet.
The study was carried out at the University of L’Aquila in Italy and the good news published by the American Heart Association in US journals Hypertension and Circulation.