Black color stool mainly indicates bleeding somewhere in the alimentary canal. Start having 2 tsf cow ghee with ginger fried in it before meals. This will heal the wounds. Have licorice tea twice daily. Take only kutaj ghan vati 2 tab twice daily after food. Stay away from heavy foods, tea, coffee, fruits, curd, non veg food etc. You can also go for vasthi karma at a panchakarma centre near you.