When blackle came out (2007) the majority of users still had CRT monitors.
There's a nice discussion of this here http://ecoiron.blogspot.com/2007/08/history-in-january-2007-mark-ontkush.html
From the above link:
There has been both praise and criticism for this initiative, with its supporters citing it as a great example of environmental thinking, and its detractors pointing out usability and aesthetic problems, as well as questions about the scientific validity of the claims. Some of the issues are listed below.
* Since the technique is most effective on CRT monitors, some proxy sites have been criticized for not mentioning this fact. In particular, the Blackle site has been heavily criticized, as it is probable that they are generating an substantial Adsense revenue stream from implementing the concept.
* CRT monitors are being phased out; about 75% of monitors in active use worldwide are LCDs. Additionally, countries with a high percentage of CRT are replacing them rapidly; for example, Display Search projects that only 18% of the monitors in China will be CRTs by the end of 2007. Therefore, although the technique would be effective for a limited period, it is questionable whether the disruption
would be beneficial.
* CRTs are generally darker than LCDs, and the text on many of the proxy sites is barely readable on monitors of this type. For example, Blackle uses a small grey font on an all black background. It is possible that these 'all black' proxy sites are only usable on LCD screens, and this would
negate the energy savings.
* Proxy sites cannot handle the heavy load that high volume sites are accustomed to. For example, on August 1st, 2007 and several prior occasions, the Blackle web server was producing intermittent error messages for extended periods of time.
So, already back in 2007, most people were questioning the usefulness for the black background web pages for saving energy.
If we were all still using CRT monitors then yes you could save a lot of energy by using the black backgrounds. But, this is 2011 and the few CRT monitors left in existence are sitting in the corner of our basements unplugged and waiting to be taken to the electronics recycling graveyard.