From Longman Dictionary of Contemporary Englishslate blue/greyslate blue/greyCCa dark blue or grey colour → slate
Examples from the Corpus
slate blue/grey• By flaking off successive layers, the tree displays a bark of beige, cinnamon, lime green and slate blue.• Matching long-line briefs, £19, s, m, l, Also in classic navy and slate grey.• Immature has tail brown but throat white, with most of bill slate blue.• The sky past his profiled head had gone slate blue above a jagged paleness of snow.• They were hard pin-points of slate blue beneath bushy eyebrows.• Most remarkably it continued to function under California's midday sun, when it's slate grey shell was too hot hold!• The falls of the flowers are a delicate yellowish green veined with slate blue. 