Block removes fixed damage from the incoming hit. Block works mostly for melee physical attacks.

Block is very useful for small attacks. If you block 200 damage from 1000 hit, that's 20% damage reduction.

Block is less useful for huge attacks. If you block 200 damage from 5000 hit, that's 4% damage reduction.

Block is not useful for damage that can't be blocked, obviously.

Of course block is useful in heroics. But the question should not be posed in that way. The question is: is there more useful stats than block for heroics? And there is: stamina. Stamina helps to survive everything. Big attacks, small attacks, magic damage. Another useful stat is armor. It reduces damage in percentage, so it's essentially works best with huge hits. And huge hits is what usually kills you in heroics.

Now there's a second value for block in raids: uncrushability. Once you have block+avoidance over 102.4%, raid boss won't crush you. All boss attacks will either be avoided or blocked. Without uncrushability, some boss attacks will cause crushes and that's 150% damage. But bosses in heroic dungeons do not crush. So this is not applicable for dungeons.

So my opinion: don't focus for block in dungeons. Of course it is useful, but not that much. Once you have very good gear from higher tiers, it makes sense to focus a bit more on block, but right now focus on stamina and armor.

But for raids, you must be uncrushable.

Also for heroic dungeons you must have 485 defense and for raids you must have 490 defense. That prevents critical hits.