I've never really seen Blue Mage tanks in action, so I can't really comment on it. I've never noticed if their spells spike hate as well as PLD, RDM, and /DRK, or if they can mitigate damage nearly as well as PLD, RDM, and NIN, but again, it's not like I've either witnessed it firsthand or looked elsewhere to draw similar conclusions. Also, I don't really consider things like Limbus and Nyzul as really "end game tanking" situations, as really, any job can tank a majority of the mobs involved in that, if not all of them.
Well, consider that BLU has AOE Flash (actinic burst), AOE stun (temporal shift), dispel (blank gaze and geist wall), sleeps--all key spells for RDM, PLD, and /DRK tanking, all with equivalent hate generation (slightly less but Conserve MP Job Trait makes up for it)--as well as a sizable hate tool noone else has, Terror (Jettatura). As well as the most MP efficient cure in the game (Magic Fruit). As well as a move that generates more hate than Invincible if it buffs 6 targets, and on a 10 minute timer (Diffusion + Exuviation). As well as MDB+50 spike spell (Saline Coat). As well as ability to cap phys. damage down without d.ring, can cap magic damage down with Shell V, MDB trait, auto-refresh, etc. etc. Oh, and lolDefense (50% defense boost from Cocoon).
The main downside to BLU tanking is that the recast timers are fairly blaaarrrrgh, but the quantity of spells makes up for it overall. And it doesn't have Shield to prevent ichi interrupts (altho flash and stun makes up for it slightly). The AOE nature of spells is a handicap too if therethings to miscast on. There isn't much in the way of enmity gear either, but haste functions well instead.
And to my knowledge BLU's tanked things such as Tiamat, Cerberus, Khim, etc.