I think this websites describes some of the problems involved in getting the perfect yorkie coat.
The progressive graying genes as well as the "dilute" gene would both influence how dark the "blue" of the coat is. Even within standard, there is a wide range of what's considered acceptable. See this page for an example,
AKC Yorkies, Yorkshire Terriers, Champion Yorkie males. The first dog, CH Blumoon He's So Hot, "Julio" has a much lighter shade of "blue" than the second dog, CH Blumoon Tuff-E-Nuff ROM "Nuff", however both are in the standard range. There are some dogs that are born black-black with out this blue quality that is often described as a gun medal grey, and there are dogs that are born that are lighter towards the more silver range. Breeders also have the problem of females who have given birth tend to lighten up, and some have brought in the black/black dog to help with this, but from what I'm told it hasn't help because the genes responsible are the blue, and the progressive graying genes as well as the dilute.
To give you an example that might help explain this, you need a blue eye gene from each parent before a human child has blue eyes, the shade of blue is determined, by the dilute gene, the eye could be a dark navy blue or a very light blue depending on the dilute genes that were inherited.
I see nothing wrong with you describing your dog as a platinum yorkie, but I do see a breeder who tries to advertise for this and sell it as something rare or special, as being unethical. Because the yorkie coat is so difficult to achieve, you are going to see much variation even among the breeders who breed to standard, but while their offspring may not all be considered standard, they will still be very beautiful. You can tell Uni has plenty of the blue gene, and probably has a silky coat. The black/black yorkies without any blue tend to have cotton coats.