Something like this had crossed my mind - but surely it can't be that simple can it? You would think that they would have done their homework and that the Blackle settings provide better "energy savings" - if indeed the savings are notable - than adjusting your own PC.
I think that unless you're using a CRT, this isn't going to make any difference. My LCD screen is blacklit, regardless of what colours it's displaying.
The optician fees will certainly counter any savings made!
I don't think that blackle is going to save you €€€€€s to be honest. I presume that they are extrapolating the potential savings from the number of users etc. Obviously small increases in energy efficiency in widely/regularly used systems could yield large total savings (e.g. Power of One and all that) but I would be a bit skeptical about this sort of thing doing it...