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 is a disorder in which your body attacks its own moisture-producing glands, such as the tear and salivary glands, notes the BBC Health site. Sjogren's syndrome is characterized by a sore, cracked tongue, dry mouth, fatigue and joint pain, as well as burning or itching of the eyes. The exact cause of Sjogren's syndrome is unknown. However, genetics and environmental factors, such as a bacterial or viral infection may play a role in its development.