Atrocoeruleus: very deep, blackish, sky-blue.
Atrophied: wasted away; unfit for use.
Atropurpureus: dark purplish, nearly black [an admixture of mauve and black].
Atrous: jet black.
Atrovelutinus: velvety black.
Atrovirens: dark green, approaching blackish [prussian green].
Attenuated: drawn out; slender; tapering.
Attingent: touching.
Atus: suffix; denotes possession of a quality or structure.
Atypic -ical: off type; not of the usual form.