Get to a holistic dentist as soon as possible. If this is a metal based crown, it will poison your body and cause you all sorts of physical issues. Please look up on google a holistic dental association to get a good crown that is not toxic made for you. Demand that the bad dentist who installed such a bad and toxic crown re-imburse you, as you were obviously not given any choice in the matter and that is illegal.
Please get a holistic doctor to install a "Diamond Crown."
If the crown is porcelain fused to metal, the black spot is probably the metal .