A BLM with a shit ton of spellspeed will ne a single target destroyer
However going for aoe... it Will be harder because you will cast back Fire III faster than your mp tick regen

Si less spellspeed = good aoe
Alors of spellspeed = good single target

Either way is fine if you know how to dance with your mana tick

And Deter is the least favorite stat for blackmage
Since both spellspeed and crit are king