Actually, there's a search engine called "Blackle" the operates, for quite some time now, on the same principle. It uses the Google technology, but in a black "skin".
I also came across this article (my apologies - it's in Hebrew. I'm sure Lior won't have a problem with it, and Ill sum, it up for those of you who don't know Hebrew): Apparently, if you have an old CRT monitor, then "black" websites do save energy, since the monitor doesn't emit any "picture" to the black areas.
In modern LCD monitors, however, it's a bit different. The monitor's light is originating from a neon lamp that's always working. Black areas are shown where the monitor's screen "filters" the light. Therefore, LCD monitors will consume less energy when displaying black websites, but the amount of energy that will be saved is smaller in comparison to that of CRT monitors.
