Specializing as a black mage serves to further strengthen class characteristics and also increases MP. Black mages are able to invoke devastating spells of ancient magic and subdue multiple enemies with a single cast.
Requirements:
Level 30 thaumaturge and level 15 pugilist.
As a thaumaturge, talk to Yayake in the Arrzaneth Ossuary (Thaumaturges' Guild), located in the Ul'dah Merchant Strip (4,5). Complete the job quest “Hearing Voices,” and receive the Soul of the Black Mage. It's on the loadstone under class and jobs