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

[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.