Blackle only really makes a dent if you have a traditional computer screen- LCD
screens have a built in back light, so it doesn't really apply for those,
but this is pretty cool!
For those worried with energy consumption and all its downsides ... When
your screen is white, being it an empty word page, or the Google page,
your computer consumes 74 watts, and when it's black it consumes only 59
Mark Ontkush wrote an article about the energy saving that would
be achieved if Google had a black screen, taking in account the huge
number of page views, according to his calculations, 750 mega watts/hour
per year would be saved.
In a response to this article Google created a black version of its search engine, called Blackle, with the exact same functions as the
white version, but with lower energy consumption, check it out.