Putting Duty Finder aside, it's for Square Enix's own rationale for why BLU isn't allow in Eureka that I don't see BLU being given free reign to go into current content via Party Finder. At least not from the beginning of an expansion. BLU right now is powerful as a DPS and flexible in its role (in the content it can access). To be frank, BLU's mediocre as a healer for actually healing, and kinda poor as a tank - but even having the ability to suddenly shift into those roles to save prog is incredibly powerful. I know this personally because BLU is my "in case of emergency, break glass" option for MINE runs progging sprouts through old content. Square Enix thus doesn't want to allow BLU into things like Baldesion Arsenal because it would be too strong.
I believe the same charge would rapidly be leveled at BLU if it could participate in current raid tiers - scaled up to max level, it's by far the most powerful and versatile DPS in the game. At 70 with an optimized spell loadout, BLU currently blows every other DPS out of the water. On top of having nearly every type of utility under the sun. There'd be outrage in the community if they just unleashed it, as is, on endgame.
Players can hardly stand having single digit deviations in rDPS between jobs, or powerful unique utility like Verraise (it's weighing down RDM DPS!) or NIN's old enmity kit (overcentralizing!). Do you really think they'd abide the existence of a job with the ability to literally change role mid-fight, or a move like White Wind that even DPS BLU can currently use at full strength, in an active raid tier? Maybe if BLU were a tank (badum-tish!), but its square is red.
So they'd need to restrict it in one of the ways mentioned above, with the most likely options being the ones that destroy BLU as it is (like a set kit). Or they'd have to nerf BLU, heavily. If such nerfs cannot be somehow constrained to max level only, then suddenly BLU's ability to clear BLU-log fights is at risk. Not to mentioned they'd need to retune the Carnivale fights too.
Basically, if BLU suddenly got a level cap increase to 90 today, I'd be very happy. That happiness would then be short lived as BLU crashes headlong into an environment it is not balanced for, with everyone's full attention on the mess, and the only way to resolve the situation either sees BLU limited again or losing its soul to become "Caster DPS #4." The best I can see for BLU is being raised to max level cap very late in an expansion's patch cycle (X.45 or later), and otherwise remaining a limited job. This would not satisfy people who want to main or raid on the job.
3/3