Blue to purple will add another stat. There are 5 versions so you will have to play the RNG to get the one you want.
After you get the purple you will have to keep making them until you crit and the purple will have the extra stat plus an open augment slot.
The only ones that will sell will be the lvl50 purples with the augment slot.
Everything else will be for you, companions, or get re-engineered for mats.