WoW Classic – Can I Block Critical Hits?
Short answer: In PvE you don’t block critical hits. In PvP you can.
In PvE, the block, critical hit, and crushing blow entries on the combat table are independent entries. When the random number is generated, the result has to be one or the other or the other.
In PvP, the case is definitely different. For example; When a rogue with Cold Blood active hits a warrior with Shield Block active, the hit must be a critical hit because of how Cold Blood works, and it’s extremely likely that the hit will be blocked given how Shield Block works.
Any white damage attack that is a critical hit cannot be blocked as well, just as in PvE. On the other hand, yellow damage attacks (heroic strike, backstab, mortal strike, etc.) that are critical hits can be blocked.