Blocking reduces a certain percent of damage. This has nothing to do with shield armor rating. The amount blocked has everything to do with skill level and perk choice, not armor rating on said shield.
The armor armor rating on a shield is added to your cumulative armor rating, or "overall" armor rating, and that rating mitigates damage in combat regardless if your shield is raised or not.
This post is correct. The things that affect your active blocking damage resistance are: Type of shield material (Daedric is better than Iron), Blocking skill, how many perks you have in Shield Wall, and any Fortify Blocking enchants you have on your gear. You can also get temporary bonuses from potions and/or the Blessing of Stendarr. Note that the Shield Wall perks operate exactly like +10% Fortify Block enchantments, i.e., if you have 3 ranks of Shield Wall, this will behave like a +30% Fortify Blocking. The description for the perk in-game is incorrect, and this has been verified through playtesting.
-Loth