Welcome to just answer.
First of all i would like to tell you that your condition is asymptomatic and thus there is nothing to worry about.
Though exact pin point diagnosis cannot be made without clinical evaluation and without looking at the spot, there are two situations that explain the situation well.
The black spot in the back of your mouth can be just a spot of calculus on the gum margin and it can be can be easily taken care by good oral hygiene practice and regular flossing.It can also be removed by your dentist by the help of professional scalers in single sitting in his office.
If nearby teeth do have metal fillings
, this discoloration can be an Amalgam tattoo,It is a bluish-gray or black discoloration present on the mucosal surface. The spots are made up of metal particles from the filling.
An amalgam tattoo does not cause any tissue damage , but the discoloration is permanent
. It will require no treatment , as this condition is harmless and therefore it can be left as it is.
Consumption of more fruits and vegetables will definitely render your gum's healthy.Fruits and vegetables also contain antioxidants which will boost your overall immune system too.
Include more of citrus fruits in your diet too that are rich in Vitamin C.They will increase the healing capacity of tissues and keep gum disease(bleeding from gum's)at a bay.Hope this answers your question. Feel free to contact me again for more information.
I wish you well!