Wiktionary
slate blue
a. Of a dark, bluish-grey colour. n. A dark, bluish-grey colour.
Usage examples of "slate blue".
Its plumage was slate blue with a powdery pinkish undercast, the topknot of four or five head feathers—.
He was still sitting in the hard seat, his face slate blue, when the front tire bumped a rock and tipped over.