To piggyback on what someone mentioned earlier (I think): BLU as a tank could be really fun if it didn't fight in the same way as the current tanks with rigid combos.
BLU could be the BRD of tanks so to speak.
It could have a priority system for damaging blue magic spells and enmity generating weapon attacks (or also spells). It could have a proc system for powerful blue magic spells like White Wind and Mighty Guard. Self-Destruct could serve as the panic button, reducing your HP to 1 while either dealing damage or interacting with the job gauge in a beneficial way - the player would then get a very short period of invulnerability (or inability to reduce HP below 1) for a healer to play catch-up and would be on at least a 420s timer to discourage use as an actual attack. Perhaps split the panic button with WW/MG and have Self-Destruct become available when the BLU is at 50% HP, resulting in a proc (could get an ability to force a proc) to either use MG+SD to withstand a massive tank buster (like PLD) or use SD+WW to pre-empt a tank buster (like DRK).