Blackle is the tougher, punk rock version of Google. Heap Media claims the darker color saves energy and that they created it for environmental reasons. See. Barflies.net is helping save the planet.
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. Since then there has been skepticism about the significance of the energy savings that can be achieved and the cost in terms of readability of black web pages.
I think they just want to show off how punk rock they are. Plus, black goes with everything.