“Blackle saves energy because the screen is predominantly black. The Image displayed is primarily a function of the user’s color settings and desktop graphics, as well as the color and size of open application windows; a given monitor requires more power to display a white (or light) screen than a black (or dark) screen”
“We believe that there is value in the concept because even if the energy savings are small, they all add up. Secondly we feel that seeing Blackle every time we load our web browser reminds us that we need to keep taking small steps to save energy”
At the time of writing, Blackle saved 131,375.030 Watt hours (according to their calculations, although an average computer takes up 125 Watts/hr). Read more about the Blackle discussion over at Wikipedia.
It is the little things that count.