It can also be caused by pepto bismal.

EDIT! The bismuth in pepto interacts with trace sulfur in your mouth and gut, staining whatever the sulfur is on. Black Hairy Tongue happens when the bismuth stains the strands of skin cells hanging onto your tongue's papillae(taste buds). A lot of food you eat has trace sulfur, anything related to onions is high in it. Turkey, eggs, any broccoli and cabbage type veggies. Anything with biotin and thiamin (vitaminB1). All have traces of sulfur.