Most cases of black diarrhea are caused by gastrointestinal bleeding. Blood will appear differently in the stool depending on the point in the gastrointestinal tract from which the bleeding originates. In this case, the bleeding usually occurs in the stomach or esophagus, and it is made black when it passes through the body's digestive juices. Bleeding from these areas may be caused by ulcers, gastritis, and excessive vomiting. Black licorice, iron supplements, and certain foods may also cause black diarrhea, as can an excess of medications, such as those containing bismuth.
The technical term for black diarrhea originating from upper intestinal bleeding is melaena. As the blood passes through the digestive system and comes in contact with digestive juices, the iron in the blood will change, leading to a black color. While the esophagus and stomach are the most common origins of bleeding, and doctors will generally look there first, it is also possible for the bleeding to start lower in the intestinal tract, such as the small bowel or colon.
Ulcers are defined as sores that form on the skin and hinder that organ's ability to function. Stomach, or peptic, ulcers are ulcers that form anywhere on the lining of the digestive system and can be extremely painful due to the acidic environment of the gastrointestinal tract. While most stomach ulcers are caused by a chronic infection of the bacteria Helicobacter pylori, they can also be caused by excessive alcohol consumption or prolonged usage of non-steroidal anti-inflammatory drugs (NSAIDs). In these cases, black diarrhea is often an indication of a stomach ulcer that has begun to bleed.
Gastritis is similar to a stomach ulcer, but is more generally defined as an inflammation of the gastrointestinal lining. Many circumstances that cause stomach ulcers may also cause gastritis, especially alcohol and NSAIDs, but gastritis may also manifest because of stress, certain autoimmune disorders, or surgery. Chronic gastritis, in particular, may go along with Crohn's disease, liver complications or kidney failure. Prolonged or severe inflammation of the stomach lining may eventually lead to bleeding and, consequently, black diarrhea.
Excessive vomiting or coughing, known specifically as Mallory-Weiss syndrome, may lead to a tear in the upper gastrointestinal tract, specifically at the juncture between the stomach and esophagus. The severe, violent retching that occurs in Mallory-Weiss syndrome is most often associated with purging eating disorders and alcoholism, but any circumstance that involves acute vomiting may lead to this condition. Generally, patients will discover the tear in their esophagus by coughing up or vomiting blood, but black diarrhea also serves as a sign of bleeding that has simply traveled in the opposite direction.
Certain foods may also cause a benign color change in the stool, such as excessive consumption of black licorice or foods with a high concentration of animal blood. Iron supplements may also cause stool to become black, or sometimes greenish, in color. Bismuth subsalicylate, a chemical most commonly used to treat digestive upset, may also lead to a harmless color change to black stool because of a reaction with sulfur present in the gastrointestinal system. In these cases, the medication or food causes the stool to turn naturally black; the diarrhea is most likely independent of this color change and is caused by another factor.