Blackout is when all projected lights are turned off, made not visible.
Depending on the console or software being used it will perform in different ways.
The simplest method is to set all DMX channels to zero, as most lighting manufacturers ensure that lamp, shutter or dimmer at zero means off, but please check your fixtures to ensure this.
Another way this is performed is by using fixture profiles so the console knows the relevant channel(s) for a given fixture so it knows how to turn off the lamp, close the shutter or set the dimmer to zero.
Most consoles have a 'Blackout' button and when it is pushed either pysically if a traditional console or mouse clicked if computer software it stays on blackout until it is pressed again to release it.