I find this idea a bit Taltos-ish -- is the inspiration "spell-eater" here?
First off we need to think about your description. Blocking a spell is not functionally different from dodging the spell -- you're looking to cause Evocation attacks to *miss*, because there's not really any sort of "hold between me and the spell" going on with Thaumaturgy. Many evocation attacks are a case of point and shoot, aimed by Discipline, dodged by Athletics. Other spells will be aimed at the mind, defended by an appropriate skill. If the skill rolls well enough -- better than the Discipline roll made to aim the spell at the target -- then the spell does not connect. It seems reasonable this item might also absorb some spell-energies even if the spell *hits*.
What you may want here is a combination of effects:
- The Item of Power rebate (which you can figure out as per the rules)
- A spell-defense power that lets you roll, say, Weapons as the skill of defense against magical attacks (stunt; new trapping; -1 refresh), probably at a +2 (stunt; -1 refresh).
- A little bit of armor that only works against magical effects. "Only against magical effects" feels narrowed enough there, so I'd say -1 refresh for every Armor:1 you wanna get, which I'd cap at 3 since Armor:3 is what you get for Mythic Toughness but we're skipping things like extra stress boxes here. Or maybe it's better to say "take a Toughness power, and give it The Catch: Only vs. Magic" and let you get all those benefits.
So, -2, reduced further by whatever your "when it hits" ability of choice is, then mitigated by the Item of Power rebate.
Not so hard.