Pepto-Bismol can certainly cause black-colored stools. The bismuth in Pepto-Bismol can combine with sulfur in your gastrointestinal tract to form bismuth sulfide, which has a black color to it. It shouldn t be anything to worry about as long as it resolves when you stop taking it. Iron pills can also make stool look black. The more serious cause of black stool is gastrointestinal bleeding. The blood gets digested in the stomach and turns into a black pigment that is excreted. Unless the black stool persists after you stop the Pepto-Bismol, you shouldn t be too concerned about it. I hope this answers your question and brings you some peace of mind.