I stopped salvaging items because I have nothing worthwhile to craft, but before that, almost all of my brimstones had come from blues.
It's worth salvaging and selling the mats on the AH. The essences sell for 1.4k+ each, and the tomes are around the same price. It's only worth selling to a vendor if a lvl 61+ items is worth more than going rate for mats.
For a good while in beta, you could get petrified bark from blues. Then they removed it from beta altogether. I don't know if it translates into lvl 60 brimstones being able to come from blues also, but I've yet to see it for myself.
Brimstones can absolutely come from blue items - I've received several myself. The rate at which you get them was heavily nerfed a few patches back, however. Prior to the patch I had gotten 4 total from blue items; since the patch that number is 1.