I think I have a cavity on a tooth that may have already had a filling. Now I have a Black stain on it.
You can get a new cavity on a tooth that had a filling. That does happen. If you feel that is what your black stain may be. Just go back to the dentist. Since you do not seem sure if they did do a filling on that tooth or not. Either way they will be able to advise you it this is new problem or something that still needs to be fixed.