Honestly, in my (Somewhat limited) experience playing BLU, I've not actually encountered anything that is reasonable grounds to justify it not being a normal (Albeit quirky) job.

Like, the main concern from people in regards to it being allowed in regular content is that "It has too many OP spells" but to be honest, most of BLU's spells just plain suck. Like, they're garbage compared to even mediocre spells from other jobs. Like, no-one's gonna be complaining about how much damage AST is pumping out with Malefic, but then you have BLU with their main filler spell for the majority of content being LESS potency than Malefic. Not Malefic II or Malefic III, plain old newbie rank 1 Malefic (Water Cannon = 120 Potency vs Malefic I = 150...)

Same thing for many other spells, many of your early level spells are just useless CC skills or stupid fluff skills that literally have no point unless you're in a party (Self Destruct and Final Sting).

Like the first actual spell that does something notable is 1000 Needles. Which some people might use as an excuse to limit party play because joining the DF for Copperbell Mines or Halatali you'd be so OP for damage output. But even then, you could just make the skill not learnable until a level where a 6s cast for 1000 damage isn't OP (Especially since you literally learn this spell from a level 26 mob that was put into the game specifically to learn this skill from. Before 4.5, Sabotender Bailaor did not exist, or if it did, it certainly didn't exist in the group of Sabotender's right next to Little Ala Mhigo)

So, in essence, they've managed to make a mediocre job, who's apparently "Unable to play well with parties" but kind of sucks at soloing. Then they limited it, so it's only useful for wasting some time collecting spells and maybe doing the Masked Carnival for a bit (If you can even scrape together enough people to go farm HM and EX spells, which I assume in a few days might become tricky as everyone gets bored of rolling the dice on RNG while grinding out those duties, either in full BLU parties, or having a single level 70 character carry some useless BLU(s) through content that the level 70 can just breeze through...)

Why? I don't even know at this point. I mean, if there was tons of OP skills or many cool but ultimately useless skills (Like RNG skills) that could maybe fool noobs into equipping them then there's actual merit to the argument that maybe, just maybe, BLU might be problematic with access to normal content.

However, as it stands, this is not the case. I mean, at this point, I question whether most people will even bother playing BLU again after they get bored at level 50 after trying to get their HM/EX skills, despite the noted like 60+ skills and increased level cap to 60 that's coming in 5.01. Since the "Job" is so shallow and not OP enough to justify all the limitations to maybe get some entertainment out of walking through level 50 dungeons and using your insta-kill skills on bosses and comboing skills together against trash...

I sincerely hope that the Devs realise how much they've ruined such an iconic job and decide to do the (Relatively minor) balance tweaks to skills in order to allow the job to participate in all content. So that not only would BLU maybe start to feel fleshed out a little with additional skills up to the max level, but also making it much, much, much less of a pain to actually go and do the duties required to learn so many of their skills by allowing use of the DF to join PuG groups to content (Whilst also, being able to scale to level 70+ would mean that one would be able to solo a majority of dungeons)