well, when people talk about true blueberry they are talking about something created by DJ short. He has created a huge number of unique strains, and primarily his work has been done with the intention of creating breeding strains. Most of these he created were not released and were used for his own work, the end result of a lot of this work were strains like blueberry.
However, what breeders look for in a seed and what those who only grow are looking for often are not the same thing. Usually those growing to smoke want a very stable strain with little pheno variance (meaning almost all the plants will behave the same, with the same dominant/recessive traits). Breeders want to cross a strain with another, and get a wide variety of characteristics from which they can choose the next generation from. This is why top notch breeders will oftentimes use different breeding systems/parents for fem and reg versions of the same strain (the assumption is that fem seeds are used by those looking to only produce bud, and reg are used by those looking to collect pollen and do some breeding).
Anyway, by saying a strain is less stable it means that the phenotype variations in the offspring can be great. A more stable strain would be one where the majority of the offspring are of the same phenotype. Breeding gets very complicated, and I'm by no means an expert, but those are the basics behind the difference between breeding strains and commercial strains.
