Black poop can be due to bleeding in the upper GI tract, including esophagus, stomach and the first part of the small intestine.
Blood can look like tar after it has been exposed to the digestive juices as it passes through the intestines. Iron
pills, pepto-bismol, and certain foods like blueberries and black licorice could make your stools black. It is in your best interest to go see a doctor.