Do you have any idea where were the polyps removed from? if they were within the biliary tree (common bile duct), then you probably had a lot of manipulation during the procedure which can make you bleed after the procedures (even days after the procedure). Blood loss in the biliary tree goes into the intestine, and then the digested blood turns black; this is called "melena". However when you do have black stools from bleeding they are usually tarry stools and very fetid.
Avoid taking any ibuprofen
or pain medications which can make you bleed. If by any chance you took Pepto-Bismol
, this is a well known cause of solid black stools.
Keep an eye on it, but I would advise you to follow up with your endoscopist ASAP and make sure everything is still stable, by checking your stools and laboratory tests.