Hello I appreciate your concern Black colour stool is suggestive of some intestinal bleeding or hard stool after prolonged constipation at times I would advise you to consult your doctor immediately for stool test for occult blood and if it comes positive than you should book an appointment with a gastroenterologist for an colonoscopy and further investigation to find out the cause for intestinal bleeding This can be caused by Peptic ulcer and inflammatory bowel disease Meanwhile please follow these guidelines Avoid constipation by taking liquid paraffin at bed time. Have lots of fresh fruits and vegetables If you fell weakness than get your blood test for hemoglobin, if it comes low Than You have to take iron and multivitamins supplements If you can provide me more information like your age and relevant medical history, I will be able to guide you better Thanks and regards