Talk to Gastroenterologist on Gastroenterologist/Black Stool

my sister is experiencing something very black in her stool for the past 2 days also lower abdominal pain around umbilical region after eating. she previously had mild antral gastritis and is currently on APD medicines. she has history of chronic diarrhoea too which is controlled at the moment. kindly tell what could be the reason. her stool test was negative for occult blood and calprotectin was 136.7, not taking iron pills.