It's time to clear this up folks. Hopefully you already know that surfing with a black background (on your computer screen) will not always save energy. If not, let me explain why black backgrounds aren't that green.
Black backgrounds can save some energy if you use a CRT screen (CRT screens are those huge and big screens). If you, like the majority, uses an LCD screen (the flat ones) a white background instead of a black will save energy.
LCD monitors are built with a light behind the screen that is always on. So when you show a white page you just let the light shine through the screen. If you show a black page on the other hand the LCD light is completely blocked and it requires more energy to shine through.
According to Google "displaying black may actually increase energy usage".
Blackle and SaveAndSearch are two of a few sites that is based on the idea that surfing with a black background on websites will save energy and therefore be good for the environment. But unfortunately surfing black isn't really that green.
If you are interested in saving energy and helping the environment there are better and more efficient ways than using black or white backgrounds when you surf or search the web.