Preferred Method for Blacking Out Level Edges in 2D Game?

Hello, I’m looking for a solution to “black out” the edges of my levels in 2D. Essentially have them fade into blackness to cover up the outer edges of sprites that form the border and cover up overlaps outside the game area.

One idea I tried was to place a flat polygon over the level and then use the painting feature to paint black where I wanted. The result was good, but it seems cumbersome because your whole level is covered up by the polygon, forcing it to be hidden in order to edit the level easily. It seems like there should be a more elegant solution for this, so I wanted to hear what you all had to say.