LCD: do black pixels spend energy?
On an LCD monitor, liquid crystal blocks light from a backlight, provided by white fluorescent tubes or, more commonly now, white LEDs. Therefore a black pixel doesn't reduce power usage in and of itself.

If a monitor is displaying all-black, most will reduce power to the backlight accordingly, primarily to produce a deeper black, and of course also to save power.

There's a question on Skeptics.StackExchange discussing this concept.