Black Mage was altered quite a but for ARR. As it is now, there is no real damage skill difference between THM and BLM. The job-specific abilities are somewhat useless utility and BLM has access to a much smaller cross-class skill pool. Freeze in 1.0 used to be a high damage attack with a hate wipe which made it valuable, for instance. Now it is a bind with an unusual cast time and limited usefulness. Convert used to swap HP and MP and Burst (which no longer exists) used to do higher damage at lower HP....which made them work hand-in-hand.

Black Mage in ARR doesn't have much that makes it superior in any way to THM and the class has more versatility. For a job that should be heavily damage focused, the job-specific skills are more defensive. I would say that may be why you don't see as many Black Mages running around as you would think.

I still have my BLM 50 in full AF from version 1.0 and although I like the THM spells, the BLM job doesn't really shine for me anymore.