Gastritis
Gastritis is an inflammation of the stomach lining. This usually happens when the mucus layer protecting the stomach lining has been eroded or made thin. This mucus layer is meant to protect the stomach lining from the harsh effects of stomach acid and other substances like certain drugs.
This inflammation may be caused by many things from infections to medications to misdirected immune responses against cells in the stomach. Certain foods and drinks that also irritate and inflame the stomach lining may cause tears that result in bleeding. This bleeding may turn your stool into the black color you observe. Gastritis causes symptoms such as bloating, nausea, vomiting, indigestion, and stomach pain. The treatment usually depends on the cause, but treating it can stop the bleeding that darkens your stool.