In a TV signal, the brightness is encoded using certain signal levels. So you'll have a set black point, and white point representing the darkest and brightest values to be displayed respectively.
If you send a signal outside those bands, it's known as a superblack/superwhite. The "blanking interval" (when the beam is meant to start at the top of the screen again) has a signal blacker than any black that would show up in a broadcast. If that were not the case, a black screen in a show would mess up your TVs tracking.
