Melena, which is the clinical term for bloody or tarry stools, is caused by problems in the digestive tract, according to the National Institutes of Health's MedlinePlus. Melena indicates bleeding in the first part of the small intestine, stomach or esophagus.
There are a number of problems that cause bleeding in the upper gastrointestinal, or GI, tract, leading to black stools, notes MedlinePlus. Some include abnormal blood vessels, bleeding ulcers in the stomach, and tears in the esophagus caused from violent vomiting, which are known as Mallory-Weiss tears. Other causes of melena include the presence of a foreign body or trauma in the GI tract, inflammation of the stomach lining or gastritis, or the blood supply being cut off to part of the intestinal tract. Overgrown veins in the stomach or esophagus, which are known as varices, may cause melena.
MedlinePlus notes that eating black licorice, taking iron supplements and taking bismuth medication can lead to black stools. Consumption of blueberries can also cause black stools.
Any changes in color of the stool should be evaluated by a medical professional, especially with black tarry stools that can be the sign of an ominous medical condition, according to MedlinePlus. Tests and studies to look for the cause of the bleeding may be undertaken, including barium studies, colonoscopies, blood studies, angiographies, stool cultures and other tests.