From what I understand, the GOOD Black Mages which you may have just missed are Terra and Rinoa, whom are apparently very effective at what they do, albeit may require their relic weapon. Big thing about both is unlike most Mages, they have a competent physical, Rinoa having range ignoring so she can use Back Row with no penalty, and Terra can use all good defensive gear making her defense pretty high for a mage (on top of both having actual HP Scores; they're averagish instead of bad.)
Looking at other factors, it seems Vanille could work as a decent Black Mage since she has 4* Black Magic and good Magic stat, even if her primary purpose is healer.
Down the line, other Black Mages who have events are Vivi and Golbez, Vivi being what appears to be a pure damage Mage, having game best Magic and that's about it for his gimmick. Golbez is a tanky mage, in that he has solid physical stats, can use heavy armor (and Axes!), but lower Magic than other mages (still solid), iffy speed, and no White Magic options.
Regarding Kain...apparently he use to be so bad they had to give him a +20 Strength boost at level 50 after an update. I think Celes and Cyan got upgrades of sorts as well, like Cyan got access to Heavy Armor when he originally didn't. ONe thing I wish there was is more documentation online. Apparently there's some more stuff released in Japan (like Terra gets a unique armor) and I can't find any stats on it, grrrr!!!