2D Mesh Lighting Effect

Hey, I'm trying to make a vision effect where the player cannot see behind walls forcing them to walk there. So far I have a top-down area built with a tile-map and am ray-casting all around the player to collect the coordinates and use them in vertices of a mesh:

My question is how do I make the rest of the screen black? Can I make the entire scene dark and somehow light up my mesh?
Thanks