Definition of "slaty"

Their faces and hands were yellow as saffron; their small and slaty eyes were set obliquely beneath lashless lids; and their thin lips, which smiled eternally, were crooked. as the blades of scimitars.

1933-03, Clark Ashton Smith, "The Isle of the Torturers", Weird Tales

But snow had now begun to fall out of a forbiddingly slaty-grey sky, and this seemed to depress both engine and crew.

1949 May and June, Cecil J. Allen, “British Locomotive Practice and Performance”, in Railway Magazine, page 192