Special effects/post processing to make a black-colored light I had this idea for an effect and I wanted to know if something like this has been done and if so how to do it:

Goal:

I want a lighting effect for an action scene where I will have an area start lighting up, while I would like to simultaneously have another area start darkening in a similar manner, all in a (relatively) lit environment. I want an effect that looks basically in all ways like a light, but its something like a black-colored light that paints its surrounding black in a light-like way.

Idea:

Basically I wanted to know if there is any way I can do some kind of post processing where I can use an LED as a green screen of sorts for the darkening effect. So wherever the LED is shining more or less maps to some kind of darkening effect/transformation.
I've got another idea, but it's a little kooky.  You light the WHOLE SCENE with spotlights - the fewer the better.  On the key spotlight, you add a gel that has an opaque black circle on it.  Then as you move that around, that section of darkness will pass over parts of the set/props/actors.

For a more focused spot, you could use a bright digital projector to project the image of a black circle on a white field.  Focus it based on the parts of the scene you're moving it across.