IF you can find one, you're getting it from a bad breeder and should run away. Good breeders don't sell puppies with disqualifying faults on full registrations; the very few times I can think of it happening it was a very specific color fault (like a merle Great Dane; merles are disqualified but they can produce harlequins and are very occasionally used in breeding) to a fellow breeder who knew what he or she was doing. That puppy would never have been sold with full registration to a relatively new owner.
Blueies are gorgeous but they do not have a "place" in Pembroke breeding. There's no reason to use them when there are so many correctly colored Pems out there.
If you're talking about a merle Cardigan (sometimes called blue), you'd begin by approaching a reputable breeder who has merles. But it should require jumping through a lot of hoops to get one on full registration.
Please don't breed specifically on a color fault and like Joanna and Beth have both stated..any reputable breeder wouldn't sell that puppy with open registration and if they did I can almost guarantee you that they are not testing for anything on their dogs and the conformations are lacking along w/empty pedigrees filled with dogs from substandard kennels.