Although most cases of diarrhea clear up within a couple of days without medication, the passage of black, tarry stools can be a sign of a more serious condition, such as an infection, pancreatitis, irritable bowel disease or colon cancer. Diarrhea with black stools requires immediate medical attention, states WebMD.
Bleeding in the upper gastrointestinal tract can cause black, tarry stools, or melena, explains Healthline. Melena results from the mixture of blood and digestive fluids. Conditions that cause bleeding in the upper digestive system include esophageal ulcers, stomach ulcers and gastritis. Abnormalities in the blood circulation within the digestive system can also lead to the formation of melena. Bowel ischemia, vascular malformation and intestinal varices are some of the conditions that alter blood flow to the gastrointestinal tract.