It may be a bit of extra work to balance, but here's how to make BLU easily work in party play without ruining it's identity:

You can go out and learn lots of skills. Then you set up to X number of skills into special slots, like role skills. Each ability is balanced (hence the work), but you can only bring X number of skills into a fight. This let's the player CHOOSE what they want to be. Do they want to be a tank? A healer? A dps? Or hybrid? The BLU job icon changes colour based on which role's skills have the most skills slotted. Majority tank-oriented skills? Blue icon.

In addition, BLU can maintain it's monster spell-castinf ability by simply getting hit by enemy attacks while a short shield is up, like with DRK or SAM. For Y seconds after, their shield skill changes to the skill they blocked, and they can cast it at will within that time window. These abilities are less inpactful as the manually learned skills - more akin to how SAM and DRK get a boost from blocking.