I stole this from reddit but honestly I think it answers a lot
To be honest, I think they can satisfy both sides of this equation.

They don't need to lock the Blue Mage from content. Blue Mage has 2 issues:

Having the correct skills.

Having balanced skills.

So the issue is that, given a Blue Mage player goes to a dungeon, or a raid, and said Blue Mage either has completely broken skills that destroy everything, or they lack the basic skills required to even contribute to the party.Basically, what we need to do is make so Blue Mages will be functional and balanced.
1. First thing you need to do is guarantee they will have all skills.

Let's assume, for example, you pick your Blue Mage from a trainer somewhere,you are unable to do dungeons through Party Finder, you can't raid, all you can do is smack monsters in the open world, be it quests, normal mobs, or FATEs.

A Lv.50 quest appears when you have all skill.

The point of the quest? Unlock all of the multiplayer content up to that point. If you want to do ARR content, which means content up to Lv 50, now you can, and only a SELECT LIST of skills will be allowed to use, up to what makes sense for a Job to have up to that point in the game.

2. How many skills would be required to make the Job functional?

Up until Lv 50, Mage DPS classes have the following amount of skills:
Black Mage: 17
Summoner: 20
Red Mage: 17 skills.
For Blue Mages who start with 49 skills, we are talking about a THIRD of the skills needing balance.
3. Balance the select number of skills you have.

These skills would be the ONLY ONES to be BALANCED.

So the idea is that before you can do the content of a said expansion or base game, you need to reach the maximum level of that expansion, and get all of the skills of that expansion, do the quest and then that will unlock the content for you.

The difference here is BLU has a unique prog system

4. How to explain the system, lore-wise?

Would make sense if, after learning all of these skills you could fight some random monsters on the field, but actually fight hard battles that require you to know what you are doing? You need a FIGHTING-STYLE.

So, the lore would be that, you and the trainer, look at all the crazy NONSENSE you have learned, and then present to you what is the REAL list of skills that you can use in the content that requires balance. Of course, all of the silly skills will still be usable in non-balanced content, which is pretty much open world content.

5. Does all this mean that I won't be able to customize my Blue Mage for certain types of content?

Yes
As for gear, thats a problem that will persist as long as the game is the way it is, hell they could have given blue scouting gear with caster role skills because why not?