I don't think BLU can function as a proper tank even with Mighty Guard when they still suffer from having the caster HP pool and significantly lower defense and also get a much heavier damage penalty then tanks do. It seems more like something you toggle on a pinch to not get melted if the MT died or if you want to mitigate something like a boss' phase transition (Diamondback is more powerful, but can't be manually disabled which makes it impractical in some situations).

Their healing is similarly more meant for pinch situations with White Wind having a huge MP cost to match its power and Infusion, well...killing you.

At the heart, it's still a DPS class.