Mob rules
While it is clear that a Blocked attack made against a player cannot be a crit, it is possible that mobs have different rules for determining whether an incoming attack is blocked or not, which includes the possibility of a "blocked crit".
It should be noted, however, that only special attacks—i.e. yellow-damage instant attacks such as Claw, Overpower, and Sinister Strike, and yellow-damage on-next-swing attacks such as Heroic Strike and Raptor Strike—have been observed as inflicting blocked crits. Normal white-damage auto-attacks have never been seen inflicting a blocked crit, and are probably subject to the same mutually-exclusive-attack-result rules that attacks made against players are.
Note too, that unlike player characters, mobs sometimes block attacks even though they have no shield equipped.
