How can I fade a stage to darkness, but certain objects are not affected?

So I'm working on a 2D sprite-based game. I have a stage planned where the stage keeps fading to darkness and the player has to press buttons to bring back the light. However, I don't want the entire stage to fade to darkness, I want some objects to stay lit. The player, lights, some enemies, etc. should not be affected (or perhaps will be affected less), but everything else will fade into darkness.

I already know one way to fade EVERYTHING to darkness using a guiTexture that covers the visible screen and messing with its alpha value, which is how I do my scene fade-ins and fade-outs at the start and end of levels. However, not quite sure how to do a fade that leaves some elements unaffected.

Some notes:

1. All of my objects are on the same sorting layer, but in a variety of orders within that layer, with objects I want to fade and objects I want to stay lit mixed all over the place, so I can't easily throw in some texture in between the objects I want to fade and the objects I want to stay lit in the sorting order.

2. I'm using some 3rd party assets I got from the Unity shop to manage parallax scrolling, and their code includes the single camera in each stage. I've played around with it before and I don't think I can change too much about the way they have the camera set up without breaking their code.

Anyone have any ideas?