Is it possible in blender to create a transparent material that would invert the light values that that pass through it. Say you had scene with a black sphere in front of a white cube. If you put a plane with this material between your scene and the camera you would then get a white sphere in front of a black cube.
Yes – use the Compositor: Let your inverting-plane have an object ID, then use that BW reference as the Factor for the Invert node. You should set your inverting-plane to be invisible (transparency of zero, e.g.) so it doesn’t show up in the initial render; it’s only influence is felt in the compositor.