I may be wrong on this being an issue with all healers but certainly for the vast majority of the healers that are blinded are always able to heal all the people when they cast their special skill. Or are affected by a skill that gives them a chance to miss a hit/special skill.
I think that if the healer is blinded there should be a chance that they will miss healing their allies because of the blindness so only one or two out of the group will get healed.
In over 15 months I’ve never seen this happen