I think what it boils down to, is whether one thinks of a black mage as an elementalist or a dark magic user. conjurer is definately the elementalist, and thaum is the dark /light magic user (but in this game we are mostly exposed to the dark magic user)
the real problem, is in their very creation, they wanted to not have the concept of i just heal and i just do dmg in these mage classes, one is supposed to work via elements the other via life and death. Now they want to make a large shift, and make them fit roles they were never meant for.
basically we cant look to lore, history and such to decide who will do what, because the truth is thaum and con both were made so they could either be a whm or a blm.
but looking at their recent changes, it seems con is leaning heavily toward whm, while its possible they could go 180 and let whm settle on thm, based on the job changes and the stat distributions and what not, it seems slightly more likely that thaum will get blm, the major thing making this a bit questionable, is the ancient magics, fairly powerful elemental nukes dont really gel well with our old concept of whm.
But who knows maybe the new whm wont be a pansypants with no elemental abilities.