Sigmar, a god famously claimed to hate arcane magic, just blessed a wizard's spell. The combination of the miracle of his holy light and shadow magic didn't explode or cancel each other out, or anything like the fundamentalist wizard haters in the Cult of Sigmar would have insisted they would, they combined to destroy the enemies of the Emperor, just like the Cult and the Colleges should. This is special and very likely completely unique. Particularly in the context of having an Elector Count and his entire army as witnesses, making it very, very hard for the fundamentalists to sweep under the carpet.
Promoting this kind of thing, the various institutions of the Empire working together, is one of the core responsibilities of the Grey College.
OOC we know it's the result of a double crit. So what. IC, that's totally irrelevant.