The spell has multiple components and utilizes scripts. Using resists seems like a quirky way to bandaid the problem.
Simply finding the magic effect that is actually being applied to the player (The one that is a dual value modifier that modifies health and stamina) and adding a GetIsID == Player != 1 is the best way to do it because this will simply prevent the player from being hit by the effect, without dealing with weird resist calculations which likely never reduce damage to zero or prevent application of an effect.
For the original idea, you would have to change the blizzard "projectile" to a script effect, make it check where the affected actor is standing, and have it then fire the actual damagin spell if the actor is not inside a specific area.
But I am confused now: Some weeks ago, I did some testing with the blizzard spell and noticed it only affects hostile targets in the area, and does not affect me.
Now when I cast it, I suddenly take damage, but it seems to be very little damage. No idea whats going on there, I was of the impression that Blizzard does not affect the player or any friendly actors in Vanilla.