I apologize that no one has responded to your question sooner. Different experts come online at various times. I just came online and saw your question. My name is ***** ***** I’m a biologist with a special interest in reptiles. would like to help with your question.
Don't try to scrape any tissue off. That can spread infection, or even introduce it where none was present to begin with. If this is not mouth rot (and it is doubtful) scarping or debriding could would the turtle and cause bleeding. These things are jobs for a vet to do under sterile conditions.
Symptoms of mouth rot include drooling of excessive saliva, appetite loss, a white, cheesy appearance to the inside of the mouth, redness of the mouth, and, as it spreads, lethargy, swelling, bone mass loss, and even death. Black on the mouth could indicate mouth rot, but it is more likely to be something else. It could be necrotic tissue, cancer, or simply an old age related change. Only a vet can tell you. These links will take you to directories of vets who treat turtles:
If you have more questions, let me know in a REPLY. I apologize again for the delay you experienced. I hope this will turn out to be nothing.