A just-published study compared the effects of dark chocolate (flavonol-rich) and white chocolate (no flavonols) on 19 persons with high blood pressure and impaired glucose tolerance. The dark chocolate reduced blood pressure by 4 points (both systolic and diastolic) and improved insulin sensitivity.
I really should test this myself. There is plenty of similar evidence.
Earlier post.
On the other hand, chocolate is said to put stress on your liver, just like alcohol does – I wonder how one would measure the liver function? Maybe by observing how tired one is?