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

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.

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.