That depends on the type of dark chocolate. A lot of the fancy dark chocolate from Europe has been Dutched or alkalized. While this process makes the chocolate easier to dissolve in liquids and gives the chocolate a darker color, it destroys most of the antioxidants. If you are eating dark chocolate for any health benefits, look at the label and make sure it hasn't been Dutched. If you are only eating for taste, it probably won't matter.
I have a bi-cuspid valve in my heart and I also have an aorta that is showing signs of a thinning wall. I have a history of aortic rupture in my family. I am monitored by my HMO for deterioration in either of those conditions which will necessitate surgery. After two years of eating dark chocolate my conditions are IMPROVING. Doctors have even said I may never need surgery.