/BLU adds some good Enmity options but suffers from a lack of not having anything by way of damage output or mitigation. Cocoon doesn't do much, really, considering how broken Defense is to start and how much defense we have already. The defense on Perle isn't bad at all, either, being at 179. That's more than Iron Ram or Valor, which used to be the standard "Defensive" sets out there, so Cocoon isn't letting us make up for using offensive choices at the expense of defensive choices.
If /BLU attack spells hit even remotely reliably, it wouldn't be such a bad thing, but as it stands, all you're getting from /BLU is enmity tools. Many of which can be interrupted if you're trying to get them off while tanking. /DNC works far better for mini-provoke + additional curing power (although, yes, it does affect damage output, as well...)
Leviathan - PLD90, BLU90, MNK 90, DRG 78