As a 44 PLD I've never had a BLM pull hate off my primary marked target, boss or otherwise, unless I am forced to apply a full 5 sec threat rotation (fast blade->savage blade->halone) on an add/secondary mob that someone unloaded on (annoyingly common, but less likely in the 40's).

I do have to fight BLM threat on 3 or 4 packs of mobs with half or more of my MP via flashes and constant tab targeting to spread my threat combo around when BLMs go AOE heavy. This can become a close competition if the BLM is level synced down for a dungeon and I'm not or they outgear me (rare). I'm sure the WARs have it easier here with more AOE taunt.

I'v always wondered, and perhaps I should level up a BLM to understand myself, are your AOE rotations that much better than your single target variations? As a PLD, a BLM that mass sleeps makes content scream by. It's almost yawn worthy how easy it is to efficiently clear a dungeon with a BLM using sleep.