Hello, my mom had an x-ray recently at the dentist and there is a black spot that showed up, the bone is healthy. no pain and behind a root canal?

Black spot: A black spot usually means an area without dense tissue such as bone or tooth. May be healing area that was damaged by the previously infected, now root canal treated, tooth. Can be nerve canal. Can be so many other things. Go with your mom to the Dentist and ask for an explanation of what your Dentist is seeing on the film. We can't see it, so review film with your mother's Dentist.