Black stool in a bowel movement can be caused by bleeding in the upper part of the gastrointestinal tract, explains WebMD. The black stool gets its color from digested blood.
Stool caused by blood in the GI tract also has a tarry or sticky texture and a foul smell, according to MedicineNet. It is a cause for medical attention. Black stool can also come from copious nosebleeds or dental operations. The blood is swallowed, and then digested.
Diseases that can cause bleeding in the upper GI tract, which involves the stomach and the esophagus, include gastritis, gastric ulcers and duodenal ulcers, says WebMD. The black stool can also be caused by cancers. Other causes of black stool include aspirin, various other blood-thinning medications and iron supplements. Bismuth medicines used to aid digestion can cause black stool. Pregnant women sometimes pass black stool because of the vitamins they ingest. Black stool can also be the result of an injury or a foreign body in the GI tract and is a sign a person has eaten lead. Some people even pass black stool after eating black licorice or blueberries. However, black stool that comes from food does not have a tarry texture.