...that I hate the Black Mage more than a goth hates himself.
Black Mage is a cruel joke. He's supposed to be this bad-ass wielder of many powerful spells. This is true 3% of the time. There's nothing I love more than having a character who has half the HP and defensive capacity of a Fighter, and the ability to do the same amount of damage as one just three times before needing to go to an inn.
At Elfland, he gets some pretty cool attack-all spells which can sometimes clear the field in a single turn. Again, he can do this about three or four times before needing to visit the inn, unless you level up way beyond the requirement for that area.
And what can the Black Mage do once those spells are exhausted? (And they almost always will be...) Stand in the back row hitting for single digit damage!
And what if you conserve the spells for when they might be needed in a boss fight? Why, same as before! Stand in the back row hitting for single digit damage, that's what! (NOTE: I'm aware that you don't conserve these spells for bosses; they're designed to make the trip to the boss easier, but once they run out you're back to attacking in the random battles anyway, and a character who dies in a boss fight after doing 3 damage isn't much fun)
It gets even better later on in the game: He gets spells that instantly kill all enemies without needing to deal direct damage, so now you can waste his MP wiping out random enemies rather than using them to help with bosses! (NOTE: Again, you won't need the MP for boss fights, but they'll still run out and leave you fighting again.)
And to top it all, the NUKE spell. An ability that deals massive non-elemental damage to all enemies. Wow, that's actually kinda useful. Too bad you have to be level 50 to use it more than three times! And at level 50 you don't need to be able to deal 300 damage to all enemies when your Fighter can deal 700 a turn! And if you've got two Fighters, NUKE is redundant anyway!
There's a silver lining behind this cloud of acid rain and whale diarrhea: Murdering Vampire in one hit with FIR3 is hilarious, and he does have access to the ever-useful FAST and TMPR spells. But wait, what's that? The Red Mage can cast those too! And he can attack for damage of more than single digits! Holy crap! Okay, so the Red Mage costs more, which is why it might be better to just bring a Thief along, who can cast FAST and TMPR later on when they're sorely needed. Man, I guess the Thief stole that silver lining.
In conclusion: The Black Mage is a rarely used liability throughout the entire game. You're probably better off with a Thief, as he upgrades to a Ninja later on, granting him access to FAST. Then there's the Red Mage, who can get this stuff right off the mark, once you can afford it all.
Anyone who's played FF5 and FFX-2 will probably tell you: Their Black Mages were utterly useless as there were stronger classes with equivalent attacks. FF9 had one, but he was soon upstaged when the White Mage learned to Summon. Black Mages look like a lame, sad rip-off of Marvin the Martian and Orko.
P.S. Lulu in FFX was actually quite useful as that game was made in a way that you had to use everyone. Go Lulu!