Black diarrhea may be an indicator of internal bleeding in the upper intestinal tract in either the esophagus or stomach, according to MedlinePlus. Black stools can also be caused by taking iron pills or eating a large volume of certain foods such as blueberries or black licorice.
Gastrointestinal bleeding can be caused by infections or ulcers in the stomach, bleeding in the esophagus, or abnormal blood vessel disorders, according to MedlinePlus. Gastrointestinal bleeding can also be caused by trauma to the abdomen.
When black stool is the result internal bleeding, the stool may also be tarry, whereas black stool caused by certain foods is usually gritty or of a normal texture, states MedicineNet. An individual should immediately call his doctor if he experiences black diarrhea, according to WebMD.