Completely black; pitch black. Can be literal (very black in color or dirty) or figurative (e.g., a dark/tainted past).
夜道は真っ黒で何も見えなかった。
The road at night was pitch black and I couldn't see anything.
トーストが焦げて真っ黒になった。
The toast got burnt and turned completely black.
作業着が泥で真っ黒になった。
The work clothes became completely black with mud.
彼の過去は真っ黒だと噂されている。
There are rumors that his past is completely shady.