I agree with Ginger....typically black diarrhea would indicate some type of internal bleeding. However, you might also want to do a search on the medications your dog is receiving.....perhaps one of the meds could be contributing to the black stool.
May I also suggest you go to the top of the page.....under the Pets section, and take advantage of Asking a veterinarian.....
Best of Luck with your dog....Connie
Black diarrhea means some kind of internal bleeding high up in the gut, I think. I am not sure what might have caused this. He may have eaten something toxic possibly if he's been eating garbage, or he could have eaten something like tinfoil, or another irritant.
Can you find another vet near to where you live who will accept a payment-plan? Or is there any way you can find any money to get him back to another vet? You need a second opinion.