Is it normal to have black stools after eating Colla Corii Asini? It is normal to have black stools because of the high content of iron ions in the gastrointestinal tract, which are oxidized and turn black after entering the gastrointestinal tract. However, in order to rule out the possibility of upper gastrointestinal bleeding, it is recommended that a stool occult blood test be performed at the hospital. If it is negative, it proves that there is no upper gastrointestinal bleeding; if it is positive, it is recommended that an electronic gastroscopy be performed to rule out the possibility of upper gastrointestinal bleeding.