Black, tarry stools can be an indication of a bleed high up in the digestive tract. This is more dangerous than seeing bright red blood in the stool.
I think that you need to return to the vet to find out what is going on. The vet might decide to scope the throat to rule out an ulcer or other problem that might be the culprit.
I can understand why you are hesitant about returning to the vet but I really think that it's necessary.
Let me know if you still have specific questions. I would be happy to tell you what I know.
