A. There can be many things that can cause staining. If the gum has receeded and the crown has metal underneathe the porcelain this will cause the margin of the crown to look darker. Ifv the crown is an all porcelain crown it could havedeveloped microleakage at the margin. This means the bond is beginning to fail, this creates a black or grey appearence at the crown margin and sometimes underneathe it.
The staining could just be staining from food stuffs,alcohol, possibly cigarettes, if you smoke.
I would go to your dentist and ask them what it could be and whether or not it can be rectified
Kind regards