Very black; pitch black. Describes something that is deeply black in color; can be used figuratively (e.g., thoroughly corrupt).
夜道は真っ黒い闇に包まれていた。
The night road was wrapped in pitch-black darkness.
古い靴が汚れて真っ黒くなっている。
The old shoes are dirty and completely blackened.
その事件の経緯は真っ黒い疑惑に包まれていると言われた。
It was said that the circumstances of that case were shrouded in deep suspicion/corruption.