Would it be possible (practical/theoretically) to have a negative color and negative bloom that would darken instead of lighten around it ?
Why not? Instead of adding bloom buffer to the frame, you need to do some remapping and multiplying.
I don’t know how bloom works so I can’t say how it relates, but you can for sure have a post effect that keeps a texture you write to from shaders* and from that you darken the final render where you need to.
*not entirely sure that’s how this all works, not my area, hehe