The Black Ghost is a short, completely black, hovering worm-like creature that can be observed through force fields at the start of many puzzles seemingly swimming through the labyrinth. The Black Ghost is capable of opening all Doors that come into its way regardless of their type or the mechanisms normally required to open them.
It is largely thought that this ghost represents the world that the character lives in, and when, in the end, the character sucks up the ghost into the Black Gun, the character is "escaping" from the world he/she once lived in (hence the title of the puzzle, The Escape). The blank white world in the end is thought to represent the destruction of the entire world, or a portal to another world. There is a Steam Forum Thread discussing the possible meanings of the ghost, and the world you enter after sucking up the ghost.