I personally play Dragoon, but of the two selected, you're SOL.
Black Mage is reliant upon Chameleon, which is a high level Archer ability, in order to be able to quickly ditch hate.
But Black Mage is clearly the best DPS of the two classes mentioned.
So level both.
This is also true. You'll want to level them both since they both use each other for cross-class abilities (though Bard doesn't have much use for Thm)
Additionally, you'll want to level Conjurer no matter what class you choose, and Monk for Second Wind, and War for Bloodbath, and Drg for Feint.
So just level them all and pick which is most fun. Most jobs can be optimized to your playstyle through gear and materia.