The black mushroom is a quest item that can be found growing in the ruins beneath Uzer. They can be used with a pestle and mortar while having an empty vial in the inventory to create black mushroom ink, which can be used to dye several items black; such as desert robes, Silverlight, origami balloons and blue wizard robes.
It can be eaten, but it will not heal any hitpoints. When eaten, players receive the message "Eugh! It tastes horrible, and stains your fingers black." This does not bear any negative side effects. The reason for this may be to give players a hint at the mushroom's intended purpose as a dye.
The name, shape, and properties of this fungus appear to be a reference to real-world ink cap mushrooms (Coprinus sp.) which are fungi that were historically used for ink or dye, hence their name and usage quests. They also tend to be inedible, though not usually harmful, which is why it heals no hitpoints in the game and "tastes horrible".