You fail. The difference in watts used by a black screen(CRT) and white screen is NOT 15watts. Also if it was I'm on Google for for no more then 10-15seconds at a time, Maybe 30seconds for every hours doing doing thing else with my computer. Using your "15watts" I would be only saving 0.125 watts an hour and maybe 1 watt a day.
I bet using Blackle will use more power because of the extra set of servers needed to be running, and the extra net bandwidth, hence more network infrastructure hardware needed. This all uses a lot of power.
I'll go as far to say if you alone where to remove all your content off of the web, and if you would turn off your computer, more energy would be save then if every one in the world started to use Blackle instead.