How can i make everything around black for a platform game ?

I want that everything will be black expect the doors and the blue platform.
The player on the left will be in the black drakness and will not be seen since it’s the first screen so when he walk through the door he will be seen on the platform.

The question is how do i make everything black but some objects ?

This is a example screenshot of what i mean dark. So everything is dark but the platform doors player and other objects:

copy this setting for your camera