... Since well the amount of damage blocked is essentialy a fixed number that is calculated by looking at the total block rating you have. But as the bosses in higher instances tend to hit a lot harder then the well fixed nr will be much smaller in comparison. Say an average mob in a heroic hits you for 4k damage and you block 1.5k out of that, which ofc is awesome. But let's say you are fighting emalon who hits for ~10-12k on heroic and you take 1.5k out of that hit, not very promising anymore is it.
Don't use shield block when the boss is doing 200% more damage since that will mean almost every strike the boss makes will be block by you for a total of around 160% more damage than normal. So you will go down very fast. (General Vezax for instance)
(Since as per normal an average tank has ~20% block / ~20% dodge and ~20% parry making 40% of the bosses strikes 0 damage to you (when you dodge/parry), making you take 40% less damage (+ ofc the ~5% extra mitigation normal blocking has). If you hit block, that will make all the bosses strikes into blocks on your end giving you 20% damage mitigation on every hit, which means you will receive 20% less damage as opposed to 40% less damage, simple math).
Blocking was useful back in the TBC days when there were still crushing blows and you needed to get a block/dodge or parry in every 2 hits not to get hit for 150% damage (a crushing blows). Nowadays as crushing blows can occur only if the mob is more that 4! levels above you, no crashing blows can occur in any lvl 80 raids as the bosses are only lvl 83 so 3 levels above you.
However you do need to stack block value since the more block value you have has the more is block by each of your block-s + the more damage your shield slam does.