Hi, Welcome to HCM. Black colored stool is known as melena which could be due to bleeding in upper gastrointestinal tract like in peptic ulcer,gastritis,esophageal varices or mallory-weiss syndrome or hookworm,drug induced etc. There may also be chances of some injury to the bowel during colonoscopy for which you are passing black stools. You should undergo routine blood count and stool examination with ultrasound abdomen will confirm the diagnosis. Treatment is mainly depend on the underlying cause but you may require antibiotics, anti-helminthic with supportive treatment like purgatives if constipation is there. Consult gastroenterologist for complete examination and further advise. Regards,