Pure breed is more a term associated with the purity of a distinct breed of any animal. In a breeders perspective, pure line dragon may be considered 'pure-bred'. Sandfire dragons may be considered a 'breed' of dragon, or even the German Giants.
In this context, pure breed would be a breed untainted by other breeds..so basically sandfire dragon bred to another sandfire dragon (hopefully from different parents). Such things are done till the phase 'breeds pure' as in, all offspring will show the same characteristics. Hence, you have pure bred huskies, or Siamese cats.
But in bearded dragons, I really don't think they have any colour morph breeding pure to my knowledge. I would certainly take any such advertising with a grain of salt...till I can learn about the actual bloodlines of the offered dragons...if they are from parents who are actualy brother and sister to eachother...from a line of other brother/sister crossings, the health wouldn't be all that reliable.