I need a bit more information in order to provide an accurate answer to your question:
-Is this black spot something that definitely appeared recently, or has it been there for as long as you can remember?
-is the spot truly black, gray, bluish, or perhaps dark brown?
-what is your racial profile-- are you black or dark-complexioned?
-do you have any similar dark spots elsewhere in your mouth, or perhaps on your skin elsewhere on your body?
-is the black spot level with the surrounding gum tissue, or is it raised?
-is the texture of the black spot the same as the surrounding gum tissue the same as the surrounding gum tissue, or is it rougher, or smoother, or ulcerated?
-are there now or have there been silver amalgam (conventional metal) fillings
in the tooth adjacent to the black spot?
-do you have any significant systemic disease, congenital anomaly, or endocrine disturbance?