Given that there's a myriad of ways to make a proper job out of BLU whilst still retaining its ability to have unique stuff such as learning new skills and accessing "OP" skills that can only be used in old content. All of which would merely require creating a single job, not 2 (Especially with additional Soul Crystals... We only have 4 slots left in the Armory chest, 2 of which will be taken up by Gunbreaker and Dancer in ShB... Adding jobs that will now need 2 different Crystals is going to be a pain).
An example for BLU would being having a "Duty Skill Set" that is automatically enabled when entering a duty that is sync'd, that has limited customization (I.e. You can swap out for different flavour of the same skills. So you can pick whichever 130 potency filler skill to use in the slot it takes but you can't remove said 130 potency spell from your spellbook entirely) so it can be balanced around while maintaining some customization. While for unsync'd content you can have all your other skill sets completely open to what you feel like using.
Essentially, acting as this "Advanced Job" dual jobs, but working within a single job. Using a framework that already exists (Being able to save skill sets for BLU) and so reducing the need to dump resources into creating and maintaining 2 jobs where one of them just won't end up being functional in a way that actually makes it feel like the job it's supposed to be.
In addition, putting in the systems to enable "Limited Jobs" to act as normal jobs also will work going into the future.
For example, if you make it so that "Limited Jobs" have to "Unlock" access to the Duty Finder. So they need to do a quest at level 50 which requires them to have spells X, Y, Z (BLU), beasts X, Y, Z (BST), mammets X, Y, Z (PUP) etc. Then they unlock level 1-50 DF. Same at level 60 and 70 and 80 etc.
Thus they're "Limited" in that they don't automatically access everything the same as a normal job, they have to meet certain requirements due to their different progression system. As opposed to being "Limited" in that they just can't participate in 90% of the content within the game and rely entirely on individually created content.