Similar to BST, BLU does need to know the Beast Strength-Weakness, basically the whole mob family eco-system. If you use a spell from a family that is weak to a mob from a family, your spell could miss/not take effect/etc. etc. Also, depending on whether your spell is magic or physically, you'll also have to deal with acc/macc, int/mnd, etc, etc. [And magical spell would also falls under the elemental strength-weakness as well]. blu magic skills affect both your physical and magical spell, etc. etc.

It's not a simple job that anyone can just pick up and play (to some extend).