I have a black spot on my tooth just below my gum line, what could this be?

Decay, tartar, stain: Or could be a tarnished old amalgam (mercury) filling. See your dentist, for a thorough evaluation so they can determine what treatment, you will need.

Calculus or decay: Sometimes calculus or tartar can look dark or black when it sits just below gum, but this could also be decay. It could also be something as simple as stain if you're a frequent coffee, tea drinker or you smoke. I'd recommend having it evaluated.

A few things: It can be a stain, cavity or discolored calculus on the tooth. See a dentist asap to have the tooth looked at and treated. Remember a small filling if that is what the tooth needs is very affordable versus a root canal. Good luck.