The principle behind Blackle is based on the fact that the display of different colours consumes different amounts of energy on computer monitors. The creators of Blackle cite a 2002 study from Roberson, who tested a variety of CRT and LCD monitors and found that an all black screen consumed less energy than an all white screen on all of them.
The creators of Blackle state that the idea behind the site came from a blog, which estimated that a black Google would save 750 megawatt-hours a year. On the Blackle homepage,
the creators of Blackle provide a count of the number of watt-hours
that they claim the users of Blackle have collectively saved.