Black poop. 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.
Serious or not much. Black "poop" comes in many varieties. Black tarry poop could be serious and related to blood in the stool and would need some immediate medical attention. If you are taking iron tablets or pepto bismol, (bismuth subsalicylate) black poop could related to this. If you are having abdominal pain or otherwise ill, consult a doctor now. If you are taking iron or pepto bismol, (bismuth subsalicylate) stop and observe your stool color.
Many causes. Melena (black, tarry stool) may be a sign of an upper GI bleed (ulcer, esophageal varices) or simply the result of food/supplement/med you've taken like black licorice, iron pills/liquid, pepto-bismol (bismuth). If none of the latter causes, then see your doc soon for full evaluation & treatment.