How did old television screens with a light grey phosphor create the darker contrast parts of the display?
Forget old fashioned TVs, have you ever watched a PowerPoint in a normally lit conference room? The projector screen is white until you turn on the projector, at which point it seems to become dark grey.
This is because your eyes are sensitive to relative light levels and not absolute. Black doesn't require no light, it's just less light. You can can make a white object dark just by putting something brighter next to it. Projectors, LCDs and CRTs of all kinds exploit this effect.