Milvaneth Sacrarium is the Temple of Nald ( life ). As players, thaumaturges join the Arrzaneth Ossuary, which worships Thal (death ).
Black Mage for Thaumaturge would make sense if the quest to gain the job came from the Arrzaneth Ossuary and if the black mage concept was divorced from elemental spells.
White Mage for Thaumaturge would make sense if the quest involves joining the sect worshipping Nald.
Black Mage as elemental Nuker makes sense for Conjurers.
My hope is that conjurers as a class are better healers than thaumaturges as a class, but white mage goes to thaumaturge to allow both mage types to perform in both a healing role and a nuking role.