You need to be comparing physical swings in the range of 20k+ unmitigated, or more, which is likely what the bare minimum will be in Ulduar. A few Naxx bosses already hit for that much. Actually, you will probably need to look at 30k swings before mitigation before you do your comparisons.
A 30k unmitigated hit would be, at 67% reduction from armor (ballpark figure raid buffed), a mere 9900 after mitigation from armor alone.
A 40k unmitigated hit would be 13200, which is a bit more in line with what we should expect. It is likely it will only be higher, and naturally, get much higher the further along in progression you go.
Because of this, Armor gets better the harder mobs hit, not worse. Conversely, Block Rating (and block value) get worse as mobs hit harder, not better.
Block Rating / Block Value gain worth in overall mitigation once mobs are hitting for smaller amounts, or hitting at extreme frequency (multiple attacks a second, AoE tanking, etc).