When does that happen? I just finished the first mission, escorting the dwarf back to radiata. This topic was covered in Ganz's tutorial, so I was expecting it from the beginning. When should I have this ability?
Volty Blasts are naturally learned after you have learned every other attack for that weapon. Like the above poster said, for the one-handed swords that is 350 kills that Jack must make. The other weapons require different amounts of kills.
It is possible to learn the Volty Blast earlier by doing the Training Dummy exercises though. One of the earlier exercises (I think its around level six but it depends on the weapon) is to unleash a Volty Blast. If you haven't unlocked the Volty Blast for the equipped weapon yet, then you receive it as a reward for beating the prior level.