Hi, Welcome to health care magic, Loose stool could be because of gut infection and black stool can come due to certain diet like chocolate fried oily foods. If black colored stool persists longer then it 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. You should undergo stool examination (since routine blood count you have already done which came normal) with ultrasound abdomen and lower intestinal tract endoscopy or colonoscopy will confirm the diagnosis. Treatment is mainly depend on the underlying cause but he may require antibiotics, anti-helminthic with supportive treatment. Continue to take probiotics as you are taking already but consult gastroenterologist for further evaluation to find out the cause. Thanks and Regards,