I like Blu for what it is now.

The only thing missing is a better structure for its 'endgame'.

I don't need Allied Seals anymore so there's very little incentive to me to do the weekly carnivale star challenges or run the log objectives. I feel that a bigger Gil reward or the latest tomestone as reward for those could be better.

Let Blue Mages solo the deep dungeons, and make a specific scoreboard for them. Also with an unique achievement, would not invalidate the challenge to do as other jobs (which would still be harder). Just this would add a TON of shelf life to the job. It's even possible to make it actually more challenging if they disable Pomanders for Blue Mages soloing them. Heck, they can even add new blue magic spells in deeper levels of those dungeons.