Though not a Summoner, I can offer a tidbit about Black Mage:
Being a Black Mage isn't uniquely locked like the White Mage is. What IS locked, however, is the Warrior of Light's protection from black magic. There is very little difference between a Thaumaturge and a Black Mage storywise. The issue is that the way a Thaumaturge channels aether.
Without the Gem of Shatotto, the Black Mage soul gem, you have a soft cap on how much aether a person can manipulate before his own inner life-aether reaches criticality and starts to get rather...
Fissile.
Boom.
Being a Black Mage isn't uniquely locked like the White Mage is. What IS locked, however, is the Warrior of Light's protection from black magic. There is very little difference between a Thaumaturge and a Black Mage storywise. The issue is that the way a Thaumaturge channels aether.
Without the Gem of Shatotto, the Black Mage soul gem, you have a soft cap on how much aether a person can manipulate before his own inner life-aether reaches criticality and starts to get rather...
Fissile.
Boom.