A Yorkshire Terriers coat is of prime importance for this breed, specifically the Yorkshire terrier colours.
A yorkie that does not have a metallic steel blue coat that lies flat on either side of its body is not recognized by the AKC breed standard.
Blue Yorkshire terriers is a term sometimes used to refer to the standard colouring of a Yorkshire Terrier. While yorkies are born with black and tan colouring, the black portion of their coat should eventually become a steel blue colour, hence the term blue Yorkshire terriers.
Other terms for Yorkshire terrier colours are blue and gold Yorkshire terriers or blue and tan for adult yorkies, and black and gold or black and tan for young pups.
An adult Yorkshire terrier must have well-defined tan markings on either side of the head and ear roots, on the muzzle, down the neck, on all four paws and under the tail.
All traces of the black and sooty puppy hairs must be completely outgrown. Any intermingling of dark hair will spoil the rich, golden tan colouring that is specific in the breed standard.