Taken from the Trivia section of wikia's Spellbreaker page:
Though it functions like a shield, Spellbreaker counts as a Heavy Armor piece. Blocking with it raises the Heavy Armor and the Block skills.
In all my 2 and a half years of playing (heavily modded) Skyrim I never friggin realized shields count towards either the light amor or heavy armor skill. I dunno... maybe it's only Spellbreaker that counts as heavy armor, or maybe there's some technicalities which I have the least bit of intrigue in. Regardless, my current character in Skyrim never, and I mean NEVER, touches skills in alchemy, illusion and heavy armor, and I would REALLY hate the fact that this realization pertaining to shield types would render this somewhat trivial quirk invalid.
Alright, after about 6 hours worth of relentless scouring the web I found this on UESP:
In Skyrim, Block skill gains are based on the amount of damage resisted when blocking, regardless of the shield used. Whether you use a Light Armor shield or a Heavy Armor shield, it levels up just the same. Also, blocking with a shield does not contribute to raising the Light Armor or Heavy Armor skill, only Blocking (you cannot Block if you want to level up the Armor trees instead).
...come to think of it, why the hell didn't i just test this out within the game? herpderp :D
I mostly use a shield to bash to stop mage casting or dragon breath attacks, and for the added armor rating if needed. If you want to level up your armor, just put the shield away. I use a light shield that I found with magic resistence for mages(until I hit the cap naturally), and I have a heavy shield for melee opponents. Wearing light armor and using alteration. I have used it to block a few times when my flesh spell has run out and need to recast it.
Armor skills do raise while blocking, it's more noticeable however if you have no blocking perks or enchantments so damage gets through. My block raised 30 points using a daedric shield, light armor raised 8.