I believe it was in the last Live Letter that they specifically addressed this. They said that because BLU spells cost so much, you'd have to devise a rotation in order to keep casting. I imagine that involves a stage where your dps tanks because you use skills that drain MP or are on a long cooldown while your MP builds again.
Which makes me laugh because that's literally BLM's playstyle. So with BLU being a limited job, we ended up getting exactly what all those doom-and-gloom sayers were saying, where it would be a BLM palette-swap.
Granted, we'll see for sure in a week, but given what was datamined and what they said in the LL, that's what it looks like to me right now