It appears that a wizard's Crit Block plays against the Crit Chance of a healing spell working on them. Of course, this should not be the case.
For example, for my level 86 Storm wizard with the following stats: Critical Rating: 125 In/Out Healing: 12%/12% Critical Block: 0
Healing Current has an 86% chance to critical and a healing of 966 on crit. This is good.
However, if my wizard were to increase its Critical Block to 27, then Healing Current has an 80% chance to critical and a healing of 776 on crit. This is bad!
Obviously, Healing Current's crit chance and healing should NOT decrease from my own Block Rating, as Block Rating should only affect enemies' Crit Chance, not my own nor my allies'.