What is the cause of black spots on the tongue?

That could be a sign of a very serious medical condition, black dots along the gum lines is known for gum cancer. Ask your dentist to take a look at it, and do it soon! Pepto Bismol can turn your tongue black and also your stool. It often scares the heck out of people. Pepto Bismol has an ingredient in it that causes this. It's used for an upset tummy, indigestion and diarrhea. This is the only cause I've found in researching "black tongue." If you have taken anything for an upset tummy or diarrhea then phone your pharmacist and ask them your question. It may well have that one ingredient that is causing your tongue to turn black.