Here's a great link to an article that can answer that well.

New advances in LCD technologies could eventually validate the belief that black is better. Newer types of LCD include a dynamic dimming capability that changes the strength of the backlight based on the image being displayed. Heap also points out that many of the new monitor technologies such as LCDs backlit with light-emitting diodes (LED), plasma screens and organic LED screens do not have a constant backlight "so we will see larger savings with Blackle as these new monitors replace the CCFL LCDs," he says.

So I take away from that it's actually quite the opposite. I mean it's a gray area but it appears as our screen technology gets better, so will the effect of blackle on power consumption.