You could post the picture here on
MedHelp. They also have expert forums on
this site, I am not sure there is one on
dermatology but there might be. For a small fee, you could have a doctor look at the picture.
In any case, I again suggest a dermatologist. Some black marks might be nothing, some might be skin cancer, and there is probably everything in between in terms of seriousness or lack of seriousness.