They found eating a few squares a day may stave off artery narrowing and hardening in smokers by countering the disruption caused by their habit.
Smoking compromises the activity of both endothelial cells, which line the artery walls, and platelets, which are involved in blood clot formation.
The research, by University Hospital, Zurich, is published in Heart.
The researchers compared the effects of dark (74% cocoa solids) and white chocolate on the smoothness of blood flow in the arteries of 20 male smokers.
Before eating 40g of chocolate, smokers were asked to abstain from other foods rich in beneficial antioxidants, such as onions, apples, cabbage, and cocoa products for 24 hours.
After two hours, ultrasound scans revealed that dark chocolate significantly improved the smoothness of arterial flow - an effect which lasted for eight hours.
Blood sample analysis also showed that dark chocolate almost halved platelet activity.
Antioxidant levels rose sharply after two hours.
White chocolate had no effect on endothelial cells, platelets, or antioxidant levels.
The researchers are convinced that the key is the high antioxidant content of dark chocolate, which contains more per gram than other food and drink laden with the substances, such red wine, green tea, and berry fruits.
Research published last year by the University of California found antioxidant-rich flavonoids in chocolate helped the blood vessels expand.
Warning
Dr Charmaine Griffiths, of the British Heart Foundation, said: "There is some evidence that when eaten in small quantities, dark chocolate might have some beneficial effects on blood vessels, but as yet no study has investigated the long-terms clinical effects - and this new, small study from Switzerland, does not change that.
"The key thing to remember about such studies is that chocolate is a bigger part of the problem than the solution.
"Whilst dark chocolate is higher in anti-oxidants, all forms of chocolate are very high in calories (typically about 500 calories per 100g) and contain an average 30% total fat.
"We are certainly not suggesting people never eat chocolate - everyone can enjoy a treat from time to time.
"But there are much better ways of improving your heart health, such as eating a varied diet, including at least five portions of fruit and vegetables a day.
"And whatever they eat, the biggest step the smokers in this study could make to reduce their risk of heart disease would be to give up."
