The Lore of Inter-breeding

[Lurina]

It's way better than 'Black Mages replaced their eye with materia', which is an idea that came from nowhere and is refuted by every single source you could possibly check.

I looked into this mystery a while ago!

The oldest mention I could find of the concept is from a early 2012 post where they describe it as a "theory" based on a cutscene where a black mage character's eye glows in a cool way. That's from the late 1.0 era.

It seems like an idea people were excited about just got mythologized into something actually in the game over the years.