I figured it would work like Fester, which reacts only to your own debuffs on the mob.
Hmm...if it's going to create database issues, then yeah we have a problem. The aim is really to placate the people who want to learn spells directly from monsters, which admittedly FFXIV's ability structure doesn't really allow.Unfortunately, I doubt the "star" system would make it into the game; the way data structures in databases work, it would probably require the kill count and target enemy tags to be applied to and then go unused for all skills on all player characters in the game.
I guess an alternative would be creating two sets of abilities (applicable only for lesser blue magic spells) and implement a hunting log of sorts (storywise this could be a monster journal that Strago kept when he was alive and you're just basically following what it says to unlock the power of your spells). Kill the prescribed mobs, and the spell is replaced by the full power version.
I know the skill you're talking about, and if people don't feel silly about BLUs being able to throw fish at their enemies, I can probably squeeze it in somehow.