Black backgrounds use less power?

At least that's the theory.... someone has done a google sponsored search with a black background thinking they'll save 750 megawatt hours a year. Blackle.com is the site. Of course, I don't know if there REALLY would be that significant a difference in consumption (margin of error in observation equipment?) I REALLY would like to know how long they estimated traffic stays at google as that would make the result vary widely anyway. If the average house used 24kwh per day (I have NO idea, just a number that seems like a decent baseline... that's 8Mwh/year... so that would be the equivalent of less than 100 houses power consumption... of course, these are just off the cuff estimates as well (just like theirs.)