More than one DPS class? (Black Mage and other)

[Marveth]

*freaking phone version of site send my message when I was typing and there is no edit button*
One time I switched from blm to smn just because on t4 we needed all different jobs to get limit in time. Otherwise it could be that I go drg because I get higher dps on that specific fight. Obviously its more advantageous to roll tank/heal but you said you had both so dps is not a bad choice, there is a situation for everything.
Personally I would recommend brd, I don't like the job but its highly useful in fights.

[Royze]

There's a huge difference in feel to all the classes.

BLM feels like a pivoted to the ground DPS.
MNK feels more like Ninja
DRG feels like DRG should - slow but agile with big numbers
PLD /yawn
WAR /doze
WHM /ragequit

[adahn]

considering shared gears factor it would be best for you to level a summoner instead because you get to use all your current caster accessories and any allagan/crystal tower body piece.
if you want to try something quite different but still a dps I recommend a melee.

[MrDiezel]

Its nice to have 1 of each class (Tank, Healer, DPS) to be versatile/useful. But @ the end of the day, play whats fun to you. We play games to have fun, not succumb to other's wishes. Thats what a Job is for.

[Lollie]

If you can't pick play the class you complement to better understand what your counterparts are doing. Mnk in this case since your bursty ranged dmg complements their high sustained close range dmg.

[Sylari]

If you're purely concerned about utility then yeah, leveling a tank or healer would get you more mileage. Having a melee DPS to compliment your caster DPS is a viable option too.

Though honestly you should probably just look at what you find fun.