I am with Sasaraixx on this one. I was thinking a spell almost like Reflect. I like the idea you have though. I support it. You could limit it by charges or even have it be like a one shot elemental blink that can be thrown up quickly with all spells of that type being on the same recast timer (think of the dancers waltzes timer/submenu or something along that line).

Example: A mob is casting Fireaga III and the black mages stun is down. Sch could pop "null-fire" and it gives the party 1 blink effect that will only block the damage from a fire type spell.

It would give a whole new depth and strategy to Scholar which, in my opinion, it lacks beyond just the "can switch between a blm and whm" for the most part. It would fit nicely in with the idea of the job as well.