You cannot Block a Critical Hit. Attacks are rolled in the following order:
Critical Hit -> Dodge -> Block -> Parry -> Normal Damage
Crit and Dodge might be swapped in order, but it has been proven by the likes of Raw Intuition and Sheltron that a Critical Hit cannot be Blocked or Parried. This is why people are slightly more amicable toward Awareness these days: it forces Crits off the table and thus slightly raises effective Block/Parry rates.