Hello Thanks for asking question at health care magic. I am Dr Laxmikant Desai. Black stool means bleeding from GI tract which is called medically melena. Do you have anemia - i.e low hemoglobin. You already have undergone upper and lower GI endoscopy. If there is no specific reason then next step would be Computed tomography of the abdomen and capsule endoscopy. Other causes of black stools are tablets like iron suppliments or sometimes infection. But if it really is melena you need further evaluation and correct management by gastroenterologist. Take care