What does it mean if my sister has a tooth thats black?

Non-Vital ?: It is very possible that the nerve in that tooth has died. Have the tooth examined by a local dentist for treatment.

Dead or non-vital : A dead, or non-vital, tooth is simply a tooth that no longer has access to blood supply. A dead or non-vital tooth is recognizable through a slight or extreme discoloration of the tooth caused by chronic abscess or root canal therapy. See your dentist for evaluation and treatment.