The black level of an image refers to the level of brightness of the darkest part of a video. It’s measured in terms of voltage, a legacy of analog broadcasting. To comply with broadcast safe specifications, the black level of a video image varies for different television standards.
It’s important to note that the black level of a video signal typically doesn’t equate to a pure black in computer graphics. Where a computer generated image typically has a black value of zero, that black level may need to be boosted to comply with broadcast safe specifications.