Darkly blue or having a dark blue color.
From Latin 'atro-' (dark, from 'ater' meaning black) + 'coeruleus' (blue). This rare descriptive term combines two Latin color modifiers to describe something that is both dark and blue simultaneously.
This word is so specialized it barely exists in modern English—it's the kind of botanical or zoological term naturalists might use to describe the dark blue plumage of some exotic bird or the color of a rare plant variety!
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