Arcana Blast was a seldom used mechanic from Arcana Heart 2 -- functioning mostly the same way. It was mapped to C+E in AH2 and you could either active it in neutral or in blockstun. In AH2, however, it would give you the passive abilities of your Arcana, which is accomplished in 3 by entering Extend Force mode, which is the reason they changed it to Extend Blast.
In AH2 it wasn't really used because the game is slower and pressure was much more manageable IMO. I think also the blow back effect that Blast had in 2 only lasted a very short time, so it wasn't that useful.
From what I've been hearing about LM, E. Blast seems very useful.