The text plays on the double meaning of 'darkroom,' which is a room for developing photographs, traditionally kept dark to prevent light from exposing the photographic materials. The humor arises from treating 'dark' as a substance that can escape when the door is opened, which is a play on the word 'dark' as both an adjective describing the absence of light and a noun as if it were a physical entity contained within the room.
Don ' t open the darkroom door ; it lets all the dark out .