I have black spot inside my mouth what causes this?

Many possibilites: See your dentist who can actually see what you are referring to and give you an accurate diagnosis.

Have it checked: There are several answers but you must have it looked at since your description was too vagus it can be a growth, a pigmentation, a pooling of blood or even an amalgam tattoo from one of your fillings, that's why you need to see your dentist.