I don't think BLU will come into competition with BST.
BLU will have much more versitality in terms of monster abilities it will be able to use. Don't need a spider in the zone to do Spider Web. No one knows yet if there will be a timer for those abilities like SMN Bloodpacts, or if it will be more akin to traditional magic, relying on a MP pool.
BSTs however, while not in control of those monster abilities, still get the monster to fight by their side. In a party, the monster is not usually used for their abilities, but first and foremost for the added damage they contribute. Downsides are the ability of the monster to uncharm, resist charms at random, and their general availability in a xp zone. Of course, BSTs can still rely on jug pets, but they are far less effective than an EM pet in the vicinity.
I do wonder however about the monster skill acquisition part. If it is indeed like previous FF titles, that means the BLU needs to be hit by a monster ability in order to learn it. That would mean for defensive abilities that the services of a BST would be required. Imagine the /shouts in Jeuno to get a BST to charm Dhalmels and have it cast Healing Wind over and over until the BLU learns the ability (I think if they were able to learn an ability 100% of the time when they get hit by it would make it too easy).
They could also make it so monsters with the needed abilities drop "scroll" items, maybe monster essences, that would allow a BLU to learn those abilities. Heh, it's still way early, all we're left with are speculations. But I'm quite excited by the possible new twists that the expansion might introduce into the game.