Is Tilly blue or silver all over, with no signs of tan? If so, she's not necessarily a blue born. She might simply be lacking the saddle tan gene that regular black and tan Yorkies have. If she has a blue nose that is distinctly lighter in color than Mimi's and Jackson's noses, then she is probably a blue born. I posted a link to some photos of blue born Yorkie puppies a few minutes ago. Here is the link, in case you missed it:
Yokie Happiness
BTW, the breeder is selling those blue born puppies as "designer" Yorkies, probably for a premium price, something that no reputable breeder in their right mind would do.