These are some excerpts from the About section of Blackle.com http://www.blackle.com/about/
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.
In January 2007 a blog post titled Black Google Would Save 750 Megawatt-hours a Year proposed the theory that a black version of the Google search engine would save a fair bit of energy due to the popularity of the search engine.
However the concept seems to too odd to accept, but it is also true that millions of drop makes a ocean.
It is yet to see that how much Blackle accomplish it motto, but what personally I think is that it's cool black display will definitely give bit of eye relief to heavy google users and computer freaks.