If someone is taking charcoal then black stools are not a concern. If they are not taking charcoal though then vary dark stooks might be an indication of blood loss through the digestive tract. This might not be serious, but might just be the result of a vitamin K deficiency or taking too much vitamin E or C, or aspirin.. It should be investigated though. Other symptoms of a vitamin K deficiency might also include nose bleeds. If one does decide to use a vitamin K supplement then be careful of the dosage. You don't want to overdo it. Besides the small amount of vitamin k in my multivitamin, I only take a vitamin k supplement when I have a nosebleed(perhaps once every several months?) or show other signs of needing it, and even then only for a day or two.