Pepto-Bismol has been known to cause one’s tongue and stool to turn black. This form of “black tongue”, however, is a result of the separation of bismuth from salicylate, forming bismuth trisulfide (Bi2S3) in combination with the sulfur found in saliva.
I just watched an episode on Dr. Oz about this and he said it could be just a stain from antibiotics and to use a toothbrush dipped in baking soda to remove. It can also be from too much Advil. Or worst cause scenario...liver damage from excessive drinking.