Ride through blizzard spell without getting hit?

A mob can resist Blizzard just like it can resist every other spell. The only difference is that the combat log doesn’t report Blizzard resists. If a mob resists Blizzard, it will not get affected by the slow.

In retail mobs can’t run through Blizzard without getting slowed because mobs can’t resist spells on retail.

Irrelevant. It’s literally impossible for an entire 40 man raid to all resist the blizzard.