Reverse Darkening Face Out
If it is done properly, it should be impossible.  If there is some amount of information there, then it might be possible to get a result by cranking up gamma or luminance, but done correctly, the information is set to black pixels for the entire area and there is no difference between pixels over that area to work from.

It is sometimes possible to reverse images that have been pixelated, blurred or distorted (or at least determine if it is the same person) but there is no way to make up information that isn't there.