So apperently someone linked to the dev team says transpose > blizard IV, never using

[Cabalabob]

Yes it's true, as long as you have the mp to cast blizzard 4 and thunder after you transpose (which in the average BLM rotation you should), you'll have enough mp refreshed to go back into your fire rotation by the time you cast both of them.

Blizzard 3 is costing you a GCD that transpose isn't, to do the same job as transpose but with some minor damage (not worth the GCD used) due to blizzard 3 damage being nerfed by astral fire. And your fire 3 damage is also getting nerfed more from umbral ice 3 than it would from umbral ice 1. Basically instead of blizzard 3, you could have done another fire 4 + transpose and gotten the same result but with more damage and then you'll get a stronger fire 3 to boot.

The difference is kind of negligible though, it could probably be made up for with a lucky crit or some good thundercloud procs and honestly wasn't worth fighting over, especially in a light farm... But the damage with transpose would technically be higher.