Normal stool should appear brown, but sometimes it comes out black. This is not normal and may be caused by several factors. These can be as benign as a person’s choice of food or as serious as intestinal bleeding. Blood in the stool (melena) often appears black instead of red because hemoglobin turns black when it passes through the intestines. Melena usually appears tarry and has a foul smell. Here are common causes of black stool:
Food and medication
Black stool can be worrying, but if you have recently consumed certain foods or supplements, it is usually no cause for alarm. Certain foods  can turn a person’s stool black. These include blood sausage, beets, dark beers, black licorice, and blueberries. Some medications , especially those containing bismuth subsalicylate, can also make your stool appear black. Taking iron supplements also has a similar effect on a person’s stool. If you continue to have black stool long after you have consuming foods and using medications and supplements that can cause it, you should see a doctor. You may have other conditions causing it.