Blue Trust

[DrWho2010]

because blu mage is as it is, the way the AI is programmed, it doesn't know what "class" you want to be, therefore it doesn't know how to react when you're in the party. even tho you're classified as a DPS class, if you want to blu tank, the AI isn't going to know that. it would require more dev cost time than it's currently worth most likely

[Karishi]

Like I said, literally just treat them as a Black Mage. I don't have a problem with not being able to Tank or be main Healer in Trusts as a Blue Mage; The Masked Carnival has already trained Blue Mages that for certain content they have to adapt themselves to the dungeon, not the other way around.
Unless there's something funky going on under the hood - and there may well be - I don't see why it would take more than adding Blue Mage to the list of classes able to use Trusts, or removing them from some blacklist. Trusts already handle ilvl and level requirements, so you'd still be limited to (at time of writing) 80 or lower content.
Or if allowing them in Trusts feels like it'll let players powerlevel their Trusts (to last year's content) at least let them use Duty Support.
My point is it hurts nobody to let them engage with the AI characters instead of having to find people, and in my opinion the task of finding people pushing people away from the system as a whole after the first couple weeks post-new-BLU-patch outweighs the social value of getting a couple people to meet via party finder because it's the only way to get their BLUes.
As a programmer I'd be shocked if the work to let Blue Mages engage with one extra system that's already fully functional - basically redefining the limitations of what "Limited Job" status means in a minor way - took more than 20 minutes of coding to achieve. And you may be right anyway; That may still be considered more dev time than BLU is worth to the team at present. But I'm still gonna post my wish.