Shield Block will grant you extra crit block during shield block, but not during the 6% parry. During the 6% parry, mastery will still be granting you the critical block it always grants you. Mastery beyond the 102.4% won't grant you more crit block than it did before the 6% extra parry, but it's still giving you crit block, making it not useless, just not as strong as it is when the combat table wasn't covered without it.
Originally Posted by Booi
If you read closely what I said there, I wasn't suggesting that the 6% parry pushes 6% more crit block onto the table when Shield Block isn't active.
Kathy, I said, "I'm lost" though I knew she was sleeping
I'm empty and aching and I don't know why
Counting the cars on the New Jersey Turnpike
They've all gone to look for America