Also known as meta room, a black box is a room where players may go during a larp to play scenes that don't fit into the here-and-now of the larp. The term is also used to describe a stand alone larp that takes place entirely in a black box.
A black box usually has an audio system and adjustable lights, making it easy to change the ambience. Players may use it whatever way they like, but it is often suggested they play out flash-backs (scenes that happened before the larp) or flash-forwards (scenes that may occur in a hypothetical future). In some cases, an organizer might be present in the black box to help players facilitate their scenes and control the sound and lights.
A player may invite any other players to enter the black box. Invitations might be made by the player whispering off-game, or by giving symbolic gifts such as a feather.
A game can also take place entirely in a blackbox, where all players are dressed in black clothes, occasionally using props and/or costumes.
Black boxes are used to enrich the stories of the characters beyond the time and space of the "main" larp, and to give players a break and an opportunity to think through their larping without having to go off-game.
Black boxes might also be used for playing/dancing a symbolic version of something that is meant to be happening in-game, such as two characters fighting or having sex.
The black box has long history in the theatre world and was (probably) introduced first as technique in Nordic larp at Swedish, En stilla middag med familjen in 2007. In 2012 the festival Blackbox CPH was started, focused on short larps using black box techniques.