Thanks for posting your query.
Black stools could indicate bleeding in the upper GI tract. An upper GI endoscopy
should be done to see if there is any source of bleeding there.Iron supplements
and pepto bismol
too can cause similar looking stool. Since you are taking iron supplements, it could be probably be due to that. Stop iron supplements and see whether your stool becomes normal. If it persists, get the endoscopy done.