The
black_HIDDEN texture is invisible in game but has a light map [i.e. lighting data]. It is therefore placed overlapping brush entities or over a skybox in order to fix lighting issues when players and NPCs walk on brush entities such as gratings or over a skybox. Placed and lit carefully under a prop model (or any entity using models), it can force the prop to be lit that specific color without the color spilling onto the world.
The texture is [not] rendered just like
NULL, so the same care should be taken to avoid creating the Hall of Mirrors effect.
This texture can be found in
zhlt.wad
.
For more information on its usage, see:
Tutorial: Models and lighting