The things with all these products that are designed to save energy is the energy being saved if any is not even being used and stored for something else. So I doubt it would make much difference anyways
The backlight on an LCD is on at full brightness all the time. Black is the result of the liquid crystals being closed rather than open. It’s quite likely that the default state of the crystals in monitors is closed (I can’t be bothered to investigate) and that some amount of energy is required to hold them open. But the energy consumption of LCD monitors is primarily consumed by the backlight, not by the draw of the crystals themselves. If you want to save energy you’ll have to turn down the backlight itself.
Only CRTs and OLEDs will save energy. Given that most displays likely to visit Blackle are LCDs, which do not benefit, the energy savings from their black theme are likely to be minimal.
Now, for some details:
CRTs work by aiming a beam of electrons at a pattern of phosphors painted on the inside surface of the glass envelope that takes up most of the space in the monitor. Energy usage is directly related to how many electrons the gun in the back has to shoot, so a dark image takes less energy to display than a light one. The bad thing is that CRTs are toxic pieces of crap which are so bad for the environment the gains of something like Blackle get lost in the noise.
LCDs work by selectively blocking the light constantly being produced by a backlight. Displaying a dark image means blocking more of the light, so there is no advantage to displaying a mostly-black screen unless the display is smart enough to turn down the backlight when most of it would be blacked out. This jibes with the experimental results posted by ChrisBooth12, above.
LED displays, such as OLED displays, are a lot of really small, really efficient light bulbs, which work by being small enough you can’t see them individually at normal distances. Displaying a dark image would indeed result in less power usage. The problem with OLED displays is that they’re currently very rare.