Most common cause of black stools or black poop is due to taking iron supplements or eating foods that are black in color.
Another common cause of having a black stool is a condition such as a bleeding ulcer.
A bleeding ulcer can let blood into your stools and can cause black poop or black stools.
Blood in the ulcer can be checked and detected through a stool or poop test.
If the black stool is caused by blood in the ulcer then it's a problem with the upper digestive tract and needs medical attention to fix.