Cor is a good buffer/DD, usually see /whm, /nin or /rng.
NIN sub is useless for COR, and WHM is just gimping yourself
RNG is by far the best sub for COR, as your accuracy will be rather sad without RNG sub. And doing damage is one part of your job.
In most parties I join, I outdamage the melees, while buffing them at the same time. NIN offers nothing other then utsu:ichi, and since you won't take hate, makes it useless. It can be usefull for pulling, but honestly, I can't remember the last time the mosnter hit me while I was pulling. They just don't have enough time to catch up when you shoot them from a long distance.
RNG sub also gives you Rapid Shot, barrage, sharpshoot, and other usefull abilities.
As for BLU, I prefer subbing /NIN over /THF. At 70+, you can deal over 1,200 damage in a matter of seconds with only 2 spells. When you sub THF, you can do the same, even when you don't use SATA. *BUT*, since you'll most likely have hate by that time, it means you'll get assraped and die unless the healer cure spams you.
/NIN also offers slightly higher TP gain, and damage output from your melees, and to optimize damage output/MP usage, 1/3 to 1/2 of your damage should come from your swords. Even as BLU/NIN, I outdamaged almost all other melees, except for like 3 that I met on my way to 75.
While /THF lets you do 900 damage death scissors, it requires you to have over 100% TP, and I could do more then that anyway with Vorpal Blade + Disseverment (or Savage Blade -> Disserverment -> Distortion, for that matter).
As for stats, BLU should mainly aim for STR while meleeing and casting spells. While yes, CHR, AGI and DEX affects some spells, STR affects all of them to a greater degree.
Most magic spells are affected by INT, except for breat-type spells that does damage depending on how much HP you have. healing spells are MND-based. There's also some magic spells that are CHR based. Although I believe that enfeebling spells are CHR based, it have not been tested thouroughly enough to say they are.
Basically, stick to STR while meleeing, as you'll get the best result that way. For a complete list of stat mods, go here:
http://www.neko-sentai.com/images/BlumageNFO.htm
Also, countrary to popular belief, TP does
NOTHING to the spells unless you use Chain Affinity, so don't save your TP pre-40.
Thought might aswell add this, /WHM is also a great sub for BLU when main healing. /BLM is currently not very explored, but I think it can be rather good for end-game once we get some more +blue magic items. Breath-type spells doesn't seem to work the same with resistance as normal spells, and there's been reports of breath spells doing damage to magic immune monsters. I haven't had a chance to try it myself, but it breath spells ignores magic resistance to some extent, it could be better then BLMes at killing stones in Dynamis =/
got her to 37. I sub bard and play a support DD, since Pama, my puppet, does most of my dmg for me. Plus, which party doesn't mind an extra song buff >: )
