OK, so, based on your experience with FFV-VI, how would you have done the blue magic? What would be the ideal that still preserves the basic idea behind it without being frustrating to play?
A fascinating question!
I guess it depends how experimental I'm willing to go with it, and also, like... I'm not a game designer. I don't know
what works, I can only take shots in the dark.
The least disruptive change I would make is this: Add to Libra the extra function that it reveals if the target knows any Blue Magic, and if it does, its name. Then add Libra to the Blue Magic list as a default spell known by all BLU. That way, you have an in-game, lore-appropriate, convenient way of checking if any enemy has something worth learning.
It doesn't fix the "you have to know that you're going to want to learn Blue Magic going into a boss fight" issue, so it's not completely ideal, but especially with FFV's flexible ability system it's a good start - you can just put Learn on your White Mage, since they also know Libra.
Going one step further, I would introduce a Coax command; its effect is that, if the target has a Blue Mage spell, it must use its next turn casting it. This means you don't need the complex setup with a Blue Mage and Beastmaster/Relm with Control, your BLU is self-sufficient, and it also has a secondary function as battle control, since you can force the enemy to use a certain spell on a certain turn. Not sure how to make it interact with blue magic like White Wind that heals the other side instead of targeting yours, though.
If I were doing a more in-depth rework, I'd just change the spread and signaling of Blue Magic spells. Instead of being a secret treasure hunt that you need luck or a guide for, blue magic spells are constrained to specific, recognizable opponents in each region and strongly signaled; so for instance you walk into a town and you read the legend of Cactuar, the living cactus that throws a thousand needles, which can be mastered. Then you know whether that's a blue spell you want and so equip your BLU going into the desert, or it's not and then you don't. The key is that I would put this signaling in focus, and everywhere. Every town has some variant of a "Hunting Board" that tells you exactly what Blue Magic is in the region, which monster has it, and at least a hint of what it does.
The truly radical alternative is Dark Souls Boss Souls. Simply put, blue magic does not in any way work the way it currently does where a smattering of bosses and random monsters have blue magic spells you must be hit by to learn. Instead, bosses (and boss-type special random encounters like the Warmech) drop a "boss soul" type item which you can then forge into a blue magic spell which corresponds to that boss's most iconic ability, and which a Blue Mage can equip/learn.