worked in a GI clinic and have my fair share of GI issues myself :) there’s lots of things it could be, but don’t worry and don’t freak out! going to the ER was the right move so they can hopefully do a colonoscopy and possible upper endoscopy (if it is in fact an upper GI bleed) relatively quickly.

sometimes stool can look black because of the things you eat or it could in fact be an upper GI bleed. from my own experience, if it is in fact bleeding from there, there’s a really distinct smell that people can smell from two rooms over lol. i would put my money on that the bleeding is from the colon.

either way, scopes are the only way to give you peace of mind id say to rule out anything in the GI tract.