My son has a history of chrons disease, for almost 10 years. Today he said he noted black arond his rectum, not stool. Is this a sign of cancer?

Depends. On what he is referring to? The black could the tarry stool which s associated with blood in the stool. If it is the tissue colored black, still notlikely to be cancer, but definitely not normal. By the way, how did you son see his own rectum?