Blm in the game is said to have control over the elements, not some elemens when you read the text.

Usually black mages were straight DD weither with over tiem spells or straight nukes. THe problem in splitting it up the way it is, is focus of the role of jobs are shifted. As the new engien rolls out, lets hoem element weakness returns. But having a WHM tossing out DD spells while the focus of buff and cure is needed can e an issue, or wasted spell space. THey can have variants, but variants that inhance the role, weither it returns hp mp or debuffs the enemy.

THey should have light based magic, and new forms of it. To level early on having a little doesn't hurt but if you choose to shift to whm, it should be a whm with white magic base, and potent enough to help in fights it would benifit.