The Illusion Specialty Bonus is a "conditional" bonus (only triggered if a concentration based spell is cast) and likely wouldn't be used a lot -- and even when it is, it just adds a small benefit. The other school specialty bonus are so much better. Please consider adding some extra bonuses / reworking this.
It's sort of balanced by the at-will ray attack that lets you blind anything up to your HD for 1 round with no save. Line up your rogues!