There has been a lot of talk and an uprising of breeders crossing Cardigans and Pembrokes and calling the crosses "blue merle pembrokes".
I have nothing against responsible breeding of mixed breed dogs but these breeders are conning buyers into thinking this cross is "rare" and charging more for merle colors...they are also claiming the dogs are AKC registerable. This is not true unless they are registered under the PAL program (and they must be spayed and neutered to be registered).
Also, the most these breeders do is test for DM. No other health testing is done, nor are the dogs being bred structurally sound.
There are LOTS of breeds and mixes in blue merle. If you want a mix, by all means buy one. But do not fall for the trap.