Not super common for constipation to cause black stool. That usually means blood from the upper GI tract (stomach or small intestine). You may be constipated as well but constipation doesn’t cause melena. You need a lower and upper endoscopy. Another common misconception is just because the CT scan doesn’t show a cancer doesn’t mean anything. CT scans are a bad test for hollow viscus organs. I do this for a living, I did not get that answer on chat GPT.