The device doesn't have to power these pixels because they are originally black in the first place. Also this gives the device a lot longer time to leave the device on.
Perfect black would be the inability for any light at all to reflect off the surface. A black hole is black because light cannot escape the gravitational pull.
Black appear black from a projector because the projector is not projecting any light at that spot.
In the "colors of light", or light emissions system, black is the absence of color, while white...