Read Yoshida's quote again.
He doesn't say "It's designed for solo play"
He says it IS for solo play. "We may as well make it into solo content you can enjoy on your own"
That is literally saying, that it, as content, can be done solo. Not "Well, most of it is solo, but you have to group up to get some spells, which is one of the primary things to do on the class..."
Perhaps if you bothered to read anything instead of picking out certain things and assuming you have an ultimate answer for it.
As I've said time and time again, the "Core skills" thing is an argument PEOPLE FOR LIMITING THE BLUE MAGE CLASS are bringing up. THEY'RE the ones saying "BLU's would get ostracized from party content because parties would want them to have certain spells"
To which, I say, that those certain spells that people would wish for BLU's to have, those "Core skills" if you would, could be streamlined into regular BLU play without affecting the tenets inherent to the job.
As far as "Where to learn them?" "How to learn them?" "How are they tied to the "limited" part?" "What happens when more skills are released?" go, if you took your head out of your ass for 2 minutes and read a full post you'd know that I suggested something akin to:
Job Quests that refer you to go and learn skills. The quest doesn't hand you the skill like with other classes doing Heavensward job quests. You still have to go and learn it just like every other BLU skill.
However, due to the necessity of having these skills (Skills which would form up the baseline of the class for group content. So, some ST skills, an AoE skill and a bit of utility), they adjust the rates for learning them to be higher, just like they've mentioned that different spells have different chances to be learned, so that they're easier to learn.
Voila, you have "Core" skills, that every BLU would be able to pick up without too much hassle or time invested that would allow them to function in content. With extra skills being luxuries that they learn of their own volition.
If new skills are released, well, if it comes with an expansion and level cap increase, new "Core" skills come from Job quests in the same way. If they're not "Core" then they just go in as regular skills they can learn if they wish.
Then, if you come back and say "Well they don't have 'Core skills'" then I'll just take that as a concession of that particular argument, the one that Yoshida himself stated:
If we made BLU a normal job with party play in mind, there was a high risk of other players forcing you to bring certain actionsIf we were to implement BLU as a normal job, there would be restrictions like "I can't raid until i learn all my new moves" or "I can't go to dungeons because i haven't learned those AoEs".
Meaning that argument is irrelevant and inconsequential for in terms of whether BLU can function as a normal class.
Honestly, the only quotes there (If they are quotes, the only quotes that I know of are just simply stating essentially "We would have to implement [BLU] in a different way to other jobs") that would suggest something radically different, is the ones referring how it wouldn't really fit into a Party.
But, that said, nothing they've shown so far nor that has been datamined has looked so unbelievably unbalancable in group content.
They really seem to be hinging on the fact that having to actively go and learn your spells is SOOOOOO bad that everyone would hate the job and people who played it.
Nothing about the class seems particularly bad:
Leveling up via Overworld? Sure, no problem. Maybe limit the class so they can't DF while leveling up, pushing them towards Overworld leveling and ensuring that BLU's end up picking up a few skills by the time they're able to join in with PuG groups.
Learning your skills? Okay, maybe a little annoying for some people, but hardly the big bad demon that they seem to be making it out as. Especially when with a bit of thinking about solutions you can come up with ways to mitigate the major problems, such as certain skills being seen as necessities and not all BLU players going and learning them before trying to enter group content, being mitigated by my aforementioned suggestion of having the job quests guide you to go learn the skills that people would consider necessities.
Multi-role capabilities? Is entirely dependant on how powerful they make the tank stance and healing spells. If they tune them correctly, they'd be useful for solo/outdated content, but the job would be just a magical DPS in groups.
Overpowered spells such as instant-death effects? Can be limited so certain targets are immune. High potency skills can be tuned down to strong, but not unreasonable levels. Status effects? Make some enemies immune.
That's about it. That's the "Unique" and "Limited" stuff from BLU. Outside of these specific things, they're literally just a normal job.