I am a connoisseur of all things Blue Mage and Blue Mage like, including Rockman. This may be a divisive answer, but the best Blue Mage class I've ever encountered was for Pathfinder. It's called Vorpal Knight, and is in the book Racial Profiles Expanded: Hungry Souls.

The main thing that sets this class apart from all other Blue Mage classes I've found is that the rest all just have a regular list of spells they can learn if they find the right monster. There isn't any open-ended exploration of possibilities; the spell list is defined in the same way a Final Fantasy Blue Mage is. Vorpal Knight instead gives you a set of criteria and rules for what KINDS and how many abilities you can copy, then turns you loose on the Monster Manual and says "go nuts", also letting you steal things like Natural Armor, resistances and immunities, movement speeds and types, etc.

If you just Google the class you'll find a bunch of seething about how overpowered it is, but their specific complaints all reveal they barely even skimmed the text and absolutely never played one. I've played one in several games and the only potential balance issues we've ever noticed is it's easy to get pretty high AC and it's technically possible to get a mechanically inconvenient number of natural attacks at higher levels. This is because you steal the attack as the monster who had it would have used it, so if you steal a Behir's Rake attack, you Rake with 6 claws. And you explicitly gain the anatomy required to use whatever attack you steal, so you gradually become a freak of nature.

https://www.d20pfsrd.com/classes/3rd-party-classes/d20pfsrd-com-publishing/vorpal-knight/

I strongly recommend at least getting a PDF of the book, since there are a fair number of support options in feats and such you'd want to at least know about.