Hi welcome to health care magic.... Here as you have history of diarrhea after milk intake so it could be lactose intolerance for which dairy product should be avoided... Here as you have black stool and loose stool since many days it is better to investigate with USG, stool culture and than if needed colonoscopy done... If colonoscopy done sample sent for histopathological examination for rule out any organic cause if present like polyp, inflammatory bowel disease etc if present... Avoid taking stress as it can lead functional bowel problem... Avoid excess spicy foods, fried foods, smoking etc... According to cause than specific treatment can be given... Ex. If infection is present than suitable antibiotic given.... Drink more water and eat more fruits... Avoid gas forming foods like onion, cabbage, cauliflower, turnip etc... Take care Advise : Gastroenterologist consultation for examination.....