Whenever something "optional" comes with Additional bonuses, it becomes Mandatory.

I had actually thought or something similar with Blue, allow them to the DF as an "extra member" but since they are called at 50, any 50 and under content would be a 5 person run, and anything 51+ blu is just too weak to really be helpful, and it only gets compounded the higher the content.

I tried duo Blu an Warrior on Shiva vs War/Whm Not curing (i wanted to see the difference) The damage that a 70 Whm can do in a single spell is 5-6 times what a Blu can do in most spells, And since the Whm is level appropriate, they don't nearly due by any stray hit.

Trust me, i like the idea of trying to incorporate Blu into the DF somehow, but i think until its on par level wise with other jobs, anything over 50 is currently out of the question. And if you limit their abilities to only things with cooldowns, its Really not worth it. Honestly, i wouldn't want to do a run as a Blu with only being able to do 1 action every 30 seconds. And thats IF you have the spells. People are still farming for those spells since they are abysmally low rates of learning.
I think they can just add Blu to the 50 and under content of DF as a DPS. I see no problem with that. Maybe just tweak a few spells, or make a few off limits when in DF, like Missile, Doom, Tail Screw, 1k needles etc, ya know, the spells that are broken low level.

I love the idea of a support role. But as said, SE mashed it into other jobs, and would be weak compared to other jobs if their only role was support.