Wiktionary
slate black
a. of a purplish black colour. n. a purplish black colour.
Usage examples of "slate black".
The slash of color almost looked like blood seeping through a gash in the slate black sky beyond the perfectly still trees.
Through the windows to his right, Brian saw the green fields of England instead of the slate black of the stormy ocean that had passed during his thankfully dreamless sleep.
Slowly, the bay went quiet again, the waters turning slate black and oily once more, a dark reflection of the shadows creeping in from the shoreline.