Black bowel movements may be indicative of bleeding within the upper digestive system, according to Mayo Clinic. If the stomach is bleeding, then a person's stools may be black.
Black bowel movements may also be indicative of taking certain medications, as stated by Mayo Clinic. Iron supplements and bismuth subsalicylate can both cause people to have black stools. Eating too much black licorice may also cause a person to have black bowel movements. Typically, having dark-colored stools is not a medical concern since stool color is largely determined by what people eat and the amount of bile in their digestive systems.