As noted, an all white web page uses about 74 watts to display, while an all black page uses only 59 watts.
While the debate rages on about whether how much more energy-efficient it is for web pages to be served on a black background instead of a white one (eg, "Black Google would save 3000 megawatts"), there is still little doubt that some energy savings would occur across the board.
My own sexy Gnome desktop has had a dark theme for quite some time now, and if you've come to the site you'll notice that the new look is very dark (but still chipper!) compared to the all-white layout I had before. I already did purchase Wind Power Energy Credits for my laptop, but the idea of saving even more energy with zero effort appeals to me.
Granted, the idea of doing anything with zero effort appeals to me.
So, if you're mulling a redesign of your own blog or website, please consider going with a dark or black theme. It's a small step but one that should have an impact on the web's energy drain if more people started going black and never going back as well.
Besides, dark skins look way better than any wussy-assed light skins out there.