blacker than black
Idiomatic expression literally meaning 'a crow on a burnt field'. Used to describe something that is already dark appearing even darker by contrast, or something standing out starkly against a dark background. Often used metaphorically for a situation where something bad seems even worse.
The phrase 'a crow on a burnt field' describes how something black appears even blacker.
His dirty hands looked even blacker, like a crow on a burnt field.