How to make an specific area fade to black

I'm trying to achieve a look where meshes are faded out into black. The player is unable to walk over to the dark areas and it's just a trick to simplify the scene. I want a look similar look to the Room:



I know I could use fog, but I want to be able to selectively apply this darkness to certain meshes (look at the cannon in image 3, it doesn't have any darkness effect). What methods can be used to achieve this effect efficiently?