Blue mages can solo most of those dungeons at the level of those dungeons, or at least, they could back before they got buffed.

They cannot coexist in the normal play ecosystem. They warm the normal play ecosystem, unless you think 100-0ing bosses in one gcd and hard ccing the entire trash of a dungeon in a mad rush to a one-shot boss room -when level synched- is okay.

You can't solve this by reducing their skills--they'll just take the best skills.

The problem is not 'blue mage doesn't have enough' when they can hit a button at level 20 that does 1000 damage when black mages struggle to hit 200 with fire.