The screen knows what color it displays at all times. It knows this because it knows what it doesn't. By subtracting what it does from what it doesn’t, or what it doesn’t from what it does (whichever is greater), it obtains a difference, or deviation. The controller board uses deviations to generate corrective commands to drive the display from a state where it does not display black to a state where it does, and arriving at a state where it displays black, it now doesn't display anything.