My boyfriend has a ceramic crown, will this cause black gums?

Cyanotic tissues come from smoking, and they can appear black or grey.  If the crown has a metal frame this may be part of the problem.  It could also be from root canal therapy if that was done as well.  Photos would help, so this is just a guess.

If he will consent to sending in a photo, this would help. If the darkening is generalized it is probably just excessive pigmentation, and nothing needs or can be done. Small areas might respond well to the Periolase ND:YAG laser to diminish the pigmentation. If it involves the gum near one or two teeth, it may be from dental fillings or if a filling was polished in the mouth, this can sometimes occur. The other possibilities would be old root canals or crown margins that are exposed.