A black desert robe is needed for the Shadow of the Stormquest. It is made by dyeing a regular desert robe with black mushroom ink. Black desert robes will not protect you from the sun and the heat of the desert. Dying the robes black makes them appear identical to the priest gown and shade robes. A set of these robes worn on a female character shows the character's midriff.
The quest requires players to be wearing at least three black garments, in order to look evil. Players may already be wearing a desert shirt (since the main part of the quest takes place in the Kharidian Desert), and black mushrooms can be found locally. This makes it a convenient choice to make the ink and use it to dye the shirt, though players will also need an empty vial with them.