Hi... Welcome to healthcare magic... According to your complain you have problem of black stool that may be because of any internal bleeding in digestive tract.. Some drugs like iron tablate cause black stool problem. Parasitic infection like giardia cause this type of symptoms. Stool examination test is helpful to detect any infection present or not.. Colonoscopy can done for further diagnosis.. Consult a gastroenterologist for further diagnosis. Detail history and investigation give exact idea. I hope my guidance helpful to you.. Take care. Thank you.