Well I found my old software.. but it does the exact same thing that the first software I suggested does. After thinking about it, I'm not sure this is possible without a hardware hack. The reason is because LCD screens require more power to make black pixels, and less power to make white pixels. The reason black uses more power is because the colors in the pixels are determined by the way the crystal is oriented in the display. More voltage means more turning, and the further the crystal is turned, the less light it will let through. The most you can do with software is make the pixels black, but even then you can't get true black without very expensive screens. To actually darken the screen, you would have to turn down the backlight, which I'm not sure is possible with software.
Anyway, here is the other software if you even want to try:
It acts exactly the same as the first, such that if you use them together, one will reset the other. This second one is easier to tweak and comes with a few presets. Worth a shot, I suppose.