What is a black sore on the inside of my jaw?

Needs to be seen: Was there an accidentally injury to the inside of your mouth? Perhaps a blood clot or hematoma? Unfortunately we can't diagnose such things without actually being able to see it. Set up an appointment with your dentist for an accurate diagnosis.

Sore: Mouth sores can be a number of things. Often they can be associated with dental or gum infections. Fungal infections can cause these or even a type of anthrax. Tumor or aphthous ulcer are possible. Get it checked.

Check with dentist: It is vital that you have the area examined and possibly biopsied. Oral cancer is treatable if diagnosed early.

Not good.: Whatever it is, it doesn't sound good. See an oral & maxillofacial surgeon ASAP, for evaluation and perhaps biopsy.