A study involving 44 middle-aged overweight men who consumed 70 grams of dark chocolate per day over two periods of four weeks, has found that dark chocolate helps restore flexibility to arteries while also preventing white blood cells from sticking to the walls of blood vessels. Both arterial stiffness and white blood cell adhesion are known factors that play a significant role in atherosclerosis.
http://www.eurekalert.org/pub_releases/2014-02/foas-wdc022714.php