Black Mage's initials would be B. M. E. because his name is Black Mage Evilwizardington, so why does Aerith say they're "B. M."?
The 'E' was lost in time?
Alternately it became taboo to say his full name so 'B.M.' was acceptable. Alternately alternately it started as an insult (the same initials mean bowel movement) and over time just became his identity. Kinda a reverse Caligula-once known as a cutesy name, now synonymous with torturous madmen in power.