I would not want to speculate as to what the black spot is without an examination.
If this is a new spot that is on your gums and does not go away, then you should definitely go back to your dentist to have it evaluated. I do not understand or agree with your statement regarding x- rays and decay. A spot on the tooth itself could be stain, decay, or from an existing silver filling. This also should be brought to the attention of your dentist.