Specifically refers to a black coat color in horses. Despite the kanji 青 (blue/green), it denotes black in this context.
That horse is a black-coated Thoroughbred.
Compound of 青 (ao, historically covering blue, green, and sometimes black/dark) and 毛 (ke, hair/fur). The exact historical derivation is uncertain, but the term is conventionally used for black-coated horses.