Black areas on gums following crowns. Dead tissue? Reversible?

I live in Cambodia and had my two top front teeth crowned around a year ago here. I am not sure if they are pure porcelain or fused with metal. Since then my gums have started to develop black areas. I visited a different dentist for a check up and he said that black areas are dead tissue resulting from the removal of nerves during the crowning. This doesn't sound right to me at all. Are my gums dead/dying? Can nerve removal result in the death of gum tissue? What can I do?