The bismuth subsalicylate in Pepto-Bismol may turn the stools and the tongue black, explains Drugsdb.com. This occurs when the bismuth subsalicylate combines with trace amounts of sulfur in the saliva and intestines. The coloring is harmless, and it disappears shortly after the Pepto-Bismol treatment has stopped.
Iron supplements, blueberries and black licorice also blacken stools, according to MedlinePlus. The color may also stem from medical issues, such as internal bleeding in the esophagus or stomach. Bleeding from these areas also causes the stool appear black, shiny and sticky, according to Cigna. A physician should be contacted if a patient notices blood in the stools.