I can't see them going and adding elemental weaknesses to everything, but you could make the elements a gameplay thing for BLU while keeping it self-contained. Have the BLU mage channel one element to the other to increase potency or gain secondary effects.
"The spark of Lightning ignites when it strikes, and thus Fire is born. The heat of Fire renders to ash all that it touches, and thus Earth is born." - Essences and Permutations.
Use of a lightning spell makes you 'Fire ready' and increases the potency of your next fire spell, the fire spell makes you 'Earth ready' and so on and so on, till you're all the way around the elemental wheel.