So I will start by saying that I absolutely agree with the devs on one thing: Block and Parry should never be able to mitigate all magic damage in the game. That just doesn't make any sense whatsoever. If you cast an AoE firebomb or Flare at someone, there is no way that they would physically be able to prevent themselves from taking that damage. However, I will posit that there are some situations where these things make a lot of sense.
The Concept: Categorizing Spells
There are, in my mind, two different categories that can be used to organize spells: direct vs. ambient, and manifest vs. ethereal. A direct spell would be something that has a travel time and that moves in a line directly from the caster to the target, while ambient is something that more or less surrounds them and deals damage from all sides. And on the other category, a manifest spell would be something that takes physical form before attacking the target, while an ethereal spell would be more along the lines of coalesced, non-solidified energy.
So something like Stone I would be a direct manifest spell, while Aero I would be an ambient ethereal spell. Determining what a given spell is would basically be done by examining its mechanics in the animation: a laser from the boss is direct ethereal, flare would be ambient ethereal, Stone III would be ambient manifest, and so on.
Blocking Direct Spells
In this case, spells that are direct would be able to be blocked. You can hold up your shield and largely stop the damage from a fireball being hurled at you, or you could stop ice shards that were being vaulted your way. But shields can only block from one direction, so block would not be able to affect something like Aero, Thunder, or Stone III, all of which surround and deal damage from all sides.
Parrying Manifest Spells
So if Shiva hurls icicles at you, you should be able to parry them, because they have physical form. You can hit ice with your sword. You would also be able to bat a stone flying your way out of the air, and if you were quick enough, preventing them from crushing you from all sides (in Stone III). But an axe cannot cleave a fireball, and nor can a sword bat away a lightning bolt. You would be able to parry attacks with physical form, but anything that was mostly volatile energy would not be able to be redirected away from you.
How Gud to Git
Because it's magic, it would not make sense for physical measures like a block or a parry to be able to mitigate them as well as they would physical damage. However, 50-70% of their physical mitigation amount would not be out of the question.
TL;DR
I believe block and parry should be able to work against some magic attacks, because it just makes sense.