
You can be a thaumaturge without becoming a black mage! Thaumaturgy itself isn't rare, and there's lots of cool stuff your character would be able to do as a trained thaumaturge alone.
Although it seems the Ossuary is the most prominent place to learn thaumaturgy, there's plenty of beastmen, bandits, outlaws, smalltownsfolk, and general would-this-person-really-have-ever-stepped-foot-in-Ul'dah's the world over who use thaumaturgy abilities. Although I couldn't tell you exactly where they learned them, it's clear that the secrets to successful thaumaturgy aren't that exclusive.
Although it seems the Ossuary is the most prominent place to learn thaumaturgy, there's plenty of beastmen, bandits, outlaws, smalltownsfolk, and general would-this-person-really-have-ever-stepped-foot-in-Ul'dah's the world over who use thaumaturgy abilities. Although I couldn't tell you exactly where they learned them, it's clear that the secrets to successful thaumaturgy aren't that exclusive.