A blackout in GoldenEye is a term for when a room's background is not loaded. Instead of seeing the normal geometry that makes up a room, there is an empty sky in its place. Objects can still be seen in blacked out rooms and collision is still loaded, meaning it is possible to play the game normally, if the player already knows the layout of the unloaded room. The name most likely comes from the fact that blackouts are most often encountered in custom multiplayer stages which use the default skies. The default skies in multiplayer are entirely black and cloudless, meaning any unloaded room will also be entirely black.
Blackouts are rare in the stock stages but do occur occasionally. Silo's fuel rooms often "blueout" momentarily when the doors leading to them are opened by guards and Bond is not nearby. Statue's unused multiplayer setup is very prone to blackouts.