Low level noise is particularly problematic for analogue video sources - true black is simply not possible on VHS because of it, and to a lesser extent on laserdisc. Properly mastered DVDs utilizing all-digital production methods are largely immune to this artefact.
macro-blocking artefacts are a potential side-effect of low level noise,
as the MPEG encoder attempts to encode the extra spurious random noise,
leaving less bits for actual picture information.
Les Miserables-The Dream Cast 19:32
Instead of being a clean, deep black, the background of this image is marred by random video noise.