My own findings after intensive testing with a thinkpad notebook are that a completely white LCD consumes measurably less power than a completely black screen.
Measurable, but far from being significant. If at all, it would result in maybe <5mins difference in runtime, with the screen running all the time. Any actual cpu work would have a far bigger impact.

If you really want to save power that way, choose a high-contrast theme that allows viewing the screen with lower backlight.