Bear, a fully tan Yorkshire Terrier, whose black/blue grew out as he grew up! Photos courtesy of Alexandra Goldy
For all Yorkies, the darker hairs will cover far less
of the body than when the Yorkie was a puppy. Present on 60 to 90% of the coat during puppy-hood, it often ends up most prevalent on the back as an adult. So, as the coat is morphing, more tan is coming in, darker hairs are falling out and fading.... and for some Yorkies, the tan overtakes the coat.
While this is considered a fault in show and Yorkies with these coats should not be bred, this is a random even that does not affect the dog in any other way aside from anesthetics (and it can't be denied that Yorkies with all tan coats are cute). And it is not caused by 'bad' shampoos or by too much brushing
or any such thing. It is not even caused by bad breeding practices (unless a breeder purposely breeds a fully tan Yorkie to another that holds mostly tan). It is simply a genetic color marker that can appear without warning and will not often even be visible until that pup matures. It can take 1, 2 or even 3 years for the black to fully disappear.
You may be wondering if darker hairs can ever grow back in. Frankly, while there may be some odd, random occurrence, in general this cannot happen.