I'm confused about how Classes/Attribute points work.

[Taieo]

So once I do the quest for my BLM I can no longer lvl my Thum? If I still can lvl the Thum past lvl 30 is there a good reason to? if I am modifying my class with the BLM job... Sorry I just don't know but have been wondering for a while...

[Squintina]

So once I do the quest for my BLM I can no longer lvl my Thum? If I still can lvl the Thum past lvl 30 is there a good reason to? if I am modifying my class with the BLM job... Sorry I just don't know but have been wondering for a while...

The BML is an upgraded version of a THM. When you unlock BLM, you get a job stone. If you equip it (while being a THM), you're a BLM, unequip it, you're back to THM. Technically, regardless of whether you're a BLM or THM, you are leveling the THM - the BLM automatically takes on whatever level the THM is and any experience earned applies to the THM (which basically means it applies to both).

Easy explanation:

If you're a lvl 30 BLM and you level to lvl 31, your THM is level 31 too. And vice versa.

[Niwashi]

So once I do the quest for my BLM I can no longer lvl my Thum? If I still can lvl the Thum past lvl 30 is there a good reason to? if I am modifying my class with the BLM job...

Squintina and Darkstride have already addressed the issue of how to switch back and forth, but as to the "is there a good reason to" question, generally not. The only thing from THM that BLM misses out on is a few cross-class skills, but the stat boosts and job specific skills you get in return outweigh those. Most people quit using the basic classes once they unlock the associated job.