However, the process of the add damage is such that any increases in Armor should magnify the effect of your existing Block Value. Armor should be calculated before Block is removed, and therefore you are simply chopping down the damage ahead of time for your Block to take effect.
For instance, say you are getting hit for 10,000 base and have 60% Armor. This leaves 4000 damage to Block away. If you have 2.4k Block, that's going to hit for ~5.2k (1600 * 3.25) or so afterwards.
If you had 65% Armor, that's 3500 damage to Block away, and then you're getting hit for ~3,575 afterwards. So 5% armor reduction can dramatically reduce the amount of damage you take from non-Critical Blocks by increasing the percentage of the base hit you block with your Block Value.
All of this would vary quite heavily on what gear you already have, though.