Refers specifically to a horse coat color: a dark gray or steel gray, often with a mix of black and white hairs. Used in equestrian and breed description contexts.
That horse has a beautiful steel gray coat.
Compound of 黒 (kuro, 'black') and 葦毛 (ashige, 'gray/roan coat'), referring to a dark gray horse coat color. The exact historical origin of 葦毛 is uncertain, but it is a traditional Japanese term for horse coat colors.