If there's no obvious reason for a black stool (such as food, a supplement, or a medication like Pepto-bismol), it could be time to look for blood in the stool. 1  A variety of medical reasons can cause black stools that are also tarry with a foul smell.
1 Lungs, kidneys or liver. Inflammation may cause pneumonia, bronchitis or other problems in your lungs; may lead to problems with kidney function; and may cause hepatitis or cirrhosis in your liver.  Lymph nodes. A small percentage of people with Sjogren's syndrome develop cancer of the lymph nodes (lymphoma).
Sjogren's syndrome. Sjogren's syndrome is a chronic autoimmune disorder. The body's immune defenses attack the salivary glands, the lacrimal glands (glands that produce tears), and occasionally the skin's sweat and oil glands. Most people with this disease are women who first develop symptoms during middle age.