I wonder if stun for the blocked hit w/this keystone would be based on the original damage or the halved damage? That could be another big difference, for the handful of people out there not playing with stun immunity
Without this keystone, block already prevents all the damage of the hit, without affecting the stun.
Preventing less damage isn't going to make it better and stun prevention - blocking just doesn't affect whether a hit stuns, it only removes the damage (or in this case, some of it).