Neither Blue Mage nor Geomancer needs to be nerfed. Beastmaster for example needed to be nerfed because for quite a time people wanted "BST only setups" and content were "requiring" BSTs in order for people to do pickup wise due to how broken it was. Did it deserve a nerf? Not really, it could have just been adjusted, but the playerbase, as always, went wild with basically playing the system and that's the end result.

Until content is BLU and GEO setups only, they don't need to be nerfed. BLU mage is the epitome of a glass cannon, it's strong but it also takes a GOOD player to actually play it to potential. Take a fresh BLU into current ilvl content, see how overpowered it is. Same with geomancer, take a Geomancer that isn't mastered into vs one that is into new content.

The "overpowered" nature of jobs are relative, the situation with BST for example was due to the nature of the job itself that literally anyone can do. Not everyone can just job change to the "OP jobs" and become the second coming,, nerfing would be pretty silly. Geomancer also would be absolutely useless if their debuffs could be resisted because it damn near negates the majority of their job mechanics.

Also, no monster in the game is "built around geomancer" unless you truly believe they went back and readjusted every single monster and content from 1999 (development) to now when they introduced geomancer.