The blood pressure effect was strongest compared with flavanol-free controls (white chocolate, milk, or a placebo pill). The average reduction was 3.7/2.7 mm Hg greater with the flavonols (P=0.002 systolic and P<0.001 diastolic).
The eight trials that used cocoa powder with low flavanol content as a control showed no significant effect of the extra flavonols from the intervention, with a mean difference in blood pressure reduction of 0.71/0.78 mm Hg (P=0.54 and P=0.30, respectively).
None of the trials looked for a clinical impact, such as a reduction in heart attacks or stroke.
Adverse effects were uncommon but included gastrointestinal complaints and distaste in 5 percent of cocoa intervention group participants and 1 percent of controls.
Limitations included the short trial durations (range 2 to 18 weeks, mean 4.4 weeks) and inclusion of some open-label trials.
"Long-term trials investigating the effect of cocoa products are needed to determine whether or not blood pressure is reduced on a chronic basis by daily ingestion of cocoa," and to determine any impact on clinical outcomes or chronic adverse effects, the reviewers concluded.
A little chocolate might do the heart good, but is no substitute for antihypertensive treatment, agreed experts contacted by ABC News in collaboration with MedPage Today.
"Flavanol is not a magic bullet," Keith-Thomas Ayoob, RD, of the Albert Einstein College of Medicine in New York City. "We need to keep this all in perspective. There are limits to what flavonols can do."
For borderline hypertensive patients, though, that small benefit might be enough along with an overall healthy diet to keep some people from needing to start medication, he noted in an email.
Going overboard could actually hurt the heart, agreed Carol Horowitz, MD, MPH, of Mount Sinai Hospital in New York City.
"I would tell [patients] to enjoy small amounts of chocolate if it's a food they like," she wrote in an email, but "that too much of it can lead to weight gain, which can increase blood pressure and introduce a host of other health problems."
Ayoob recommended "no more than an ounce a day, for sure, and cocoa powder is probably best...as it delivers the most flavonols with the least amount of calories."
In related news, a study released this week reported that dietary flavanols may improve cognitive function in elderly subjects with mild cognitive impairment.