Black light

Is it possible to make a light that… darkens?
A negative light, in other words.
I suspect and edit to the engine’s code is needed to do this, can somebody please confirm?

i tried to do something like that while ago, by inverting the shader custom lighting, then assign the modified shader to objects:

Mhhh I see effect in the shadows of the plane, but the object (sphere) still seem to be lighted normally.

probably had the shader assigned on ground plane only