Jet-black means a very deep, shiny black — as dark as jet, a hard black stone people once used for beads and jewellery. It is a colour word, not a darkness word: you say jet-black hair, a jet-black car, jet-black feathers. Pitch-black is the cousin people mix it with; that one is more about a room or night so dark you cannot see. Jet-black can still have a little shine, like polished stone. Everyday talk often just says really black; jet-black sounds a bit careful, even a little literary, the kind of thing you write or say when you want the colour to feel rich. Always the hyphen in careful writing. And this jet is not the aeroplane: same spelling, different story.