He hasn't had any vaccinations since April, so hopefully we're out of the woods as far as vasculitis caused by vaccinations. How much time was there between your dog's vaccination and the appearance of black skin?
I feel like it's encouraging that he has absolutely no other symptoms. He's eating well, still acting like a puppy, and has no hair loss. I hope the vet will be able to determine what's going on. I made an appointment with the one who has been in practice for the longest (more than 10 years)... hopefully she's seen this before and will know what to test for.