The darkest stool in a healthy human is **black.** It can result from eating **black licorice, blueberries, blood sausages, Oreo cookies** or **grape juice.**
Other foods that may cause almost black stools: dark chocolate, beets, cranberries, prunes, dark green leafy vegetables, such as spinach or kale.
They are not nutrients, but natural or artificial dyes in the food, that can turn stool black.
Black, tarry and smelly stool is suspicious for bleeding from the upper gastrointestinal stomach (from mouth to stomach), since the acid in the stomach turns blood black.
Sources: (Medline Plus, OU Medicine, 25Doctors.com)