It's four or five purples for each blue. And you definitely do not have to get every single schematic as a lot of them are not worth much.
- cunning and ANY defensive stat: no cunning tank
- any redoubt with anything but shield: tanks do not want any other stat
- any non-redoubt with shield: again no tank wants any other stat and non-tanks don't want defensive stats
- any presence: you're better served gearing up your companions than gearing yourself to buff your companions
If you get these while going for more worthwhile schematics, so be it, but they won't sell.
Unfortunately those are the ones I keep getting when I'm looking for the one that has less buffs overall but have a bunch of power and surge.
I actually crafted 25 of the Level 54 Blue STR/END earpieces last night then RE'd all of them and got 0 purples.
ARGH............thankfully Frasium is dirt cheap.