I feel that BLU would work best as a tank class, have some of the permanent monster/beast abilities learned from job quest beasts/monsters while also providing a skill that allows them to get monster/beast ability charges they could use for later.

For instance they could use a beast ability from their job quest that renders them temporarily immune to status effects for a short while so they could intentionally take a Malboro Bad Breath attack, granting them a Bad Breath charge they could save to use later on a pack of mobs. Using the inflicted stack of debuffs as a form of defensive cooldown, seeing as it inflicts blind, paralyze, etc. With a later high level ability they could unlock being the bad breath attack so they do not have to go out and pick up a charge to take into the dungeon.