You say this now, but knowing people the way I know them, this is more likely:
BLU: Hey Duelle, let me tank the next giant Beetle so I can learn Rhyno Attack from it.
Me: Erm...you're a mage, with a mage-level HP bar. You're gonna get 3 shot...
WHM: Just hurry i wanna get thise over with.
Other DPS: I want my tomez nao!!!!!!
Bickering ensues, etc, etc.
I see what you're getting at, but that's apples and oranges. Summoners are a special case given where their abilities and power come from. They still follow the 18-ability rule (because it's a job built atop Arcanist), and their job abilities outside of Summon III are still following the design paradigms set for every job.
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I think the discussion hasn't really set any sort of rules in particular. For example, I'm assuming BLU will be unlocked as a lv30 job the way it went for DRK, MCH and AST. That means that the unlock quest will likely involve you equipping the Soul of the Blue Mage and getting a quest tuned for a lv30 BLU to complete and unlock your first job ability. As we saw in unlocking the Heavensward jobs, you equip the crystal and all of a sudden you get 30 levels worth of abilities. That's where the problem stems from for me, and my preferred solution is having the BLU start out with abilities learned, but in a weak/incomplete stage. Strong enough to clear the unlock quest but a far cry from ideal should the BLU want to afterwards queue for duty finder.
My timeline would go as follows:
1) Unlock quest: Circumstances cause you to equip the BLU job crystal.
2a) My suggestion: You gain the standard faire of abilities a lv30 job would have. These abilities are weak but enough to get you through the quest, after which you'd have to go and kill the prescribed number of mobs to get yourself caught up.
2b) Alternative suggestion: You gain only the very basics (following my write up, this would mean you only have Magic Missile, Efflux, Magus Fist and Lancet upon equipping the BLU crystal) and the unlock quest itself has you killing a bunch of different mob types to populate the "empty" 1-30 ability slots.
3) The unlock quests ends with you fighting an Archaeodaemon that after defeating you absorb to learn you first Greater Blue Magic spell, Lifebreak.