Hi and welcome to Healthcaremagic. I understand your concerns and I will try to help you as much as I can. Black stool may be sign of this vitamin that you were taking or sign of certain diet. But also this can indictae much worse issues such as gastrointestinal bleeding from stomach ucer for example so you should do gastroscopy to evaluate it more accurately. You need to avoid fried food, carbonated drinks, coffee, alcohol and spicy food. You should eat more milk products, vegetables and boiled food. I hope I have answered you query. If you have any further questions you can contact us in every time. Kindly regards. Wish you a good health.