There have been a couple of scientific studies on the potential health benefits of dark chocolate reported recently:
Dark chocolate consumption improves leukocyte adhesion factors and vascular function in overweight men (Mar 2014), The FASEB Journal, 28(3), 1464-1473
The effect of acute dark chocolate consumption on carbohydrate metabolism and performance during rest and exercise (2014), Applied Physiology, Nutrition, and Metabolism, 39(2): 173-182
Dark chocolate and vascular function in patients with peripheral artery disease: A randomized, controlled cross-over trial (Feb 2014), Clinical Hemorheology and Microcirculation
These studies are, in general, small scale and the results are somewhat ambiguous. But I don’t need to wait for the science to catch up and I’m starting a supplementation routine now.