The Cochrane Group's report said chemicals in cocoa, chocolate's key ingredient, relaxed blood vessels.
The theory is that cocoa contains flavanols which produce a chemical in the body called nitric oxide. This 'relaxes' blood vessels making it easier for blood to pass through them, lowering the blood pressure.
However, studies have thrown up mixed results. The Cochrane analysis combined previous studies to see if there was really an effect.
A systolic blood pressure under 120mmHg (millimetres of mercury) is considered normal. Cocoa resulted in a 2-3mmHg reduction in blood pressure. However, the length of the trials was only two weeks so the longer term effects are unknown.
Lead researcher Karin Ried, from the National Institute of Integrative Medicine in Melbourne, Australia, said: "Although we don't yet have evidence for any sustained decrease in blood pressure, the small reduction we saw over the short term might complement other treatment options and might contribute to reducing the risk of cardiovascular disease."
Dark or milk?