Black goblin mail is made by crushing black mushrooms into an empty vial and then using the resulting black mushroom ink on goblin mail. Like all goblin mail, it is too small for players to normally wear as armour.
However, during the Land of the Goblins quest, the player can turn into a goblin using a transmogrification potion to access the tribal areas of the Goblin Cave. While wearing black goblin mail, the player can enter the Huzamogaarb goblin tribe's area and pickpocket the Huzamogaarb priest to obtain the Huzamogaarb key required to continue the quest.
Black goblin mail cannot be traded, and therefore has no value. It can only occupy space in the inventory or bank. However, the brown goblin mail is tradeable.