Or...you could have faith that the devs are bright enough to realize that Black Mage can't be effective on battles like Titan and Ramuh given their spell-set limitations and create other battle mechanics within the fight that still make BLM's useful...just not in a direct manner.
-Smaller pack of mobs that spawn during the battle Mobs that need slept
-Smaller pack of mobs that spawn during the battle that need aoe'd
-Machines or devices that need 'charged' with thunder magic to aid in battle
...etc
Also, the actual Elemental damage each type of spell puts out is far less important in this game than what the mechanics behind the spell allow it to do...especially when cast in a combo.
Case in point: Mistress fight.
She's weak to fire yet lighting is the more effective spell.
Lighting magic is SUPPOSED to be the better choice for comboing on Single-target Mobs. When cast in combo, the combo bonuses even help to raise your damage.
Fire magic is typically for groups of weaker mobs that need quick AOE damage.
Ice magic is typically 'defensive' cast spells for the Black Mage. (this is also why in 1.0 the ice spells tended to do less damage as less damage=less hate)
During 1.0 SE changed how the whole Black Mage thing works. In past games, Elemental Affinity meant everything...now the modifiers are more important. Sure, there are exceptions...it would be a terribad idea to cast Lighting on Ramuh, but overall, even if a normal mob is resistant to a specific spell you can still kill it with exactly that spell.
Lastly, if this is just a 'BLM needs spells just in case they have to face an elemental resistant mob' the more prudent path would be asking for an 'Umbral' DD spell and not an Elemental, as then you would always have at least one spell you could cast no matter the mobs elemental resistance.
As you said yourself:
Well the same could be said of Dark/Umbral-based spells. So if anything ask for that, and leave CNJ/WHM out of it.