Yes, BLU is very capable of tanking a great many things and probably able to tank current End-Game content as well. However, BLU's tanking in some form relies heavily on spell casting; usually making some fights tricky if the NM keeps spamming TP. This is likely the reason that the times I saw Spicyryan tanking he was either disengaged or PASSIVELY engaged.
What it all boils down to is that it won't be quite as well suited for it as PLD or RUN as any attacks that made it passed BLU's impressive defenses would hurt it more than it would the RUN/PLD. Add onto that, the fact that a lot of NMs have buff erasing moves too. BLU is considerably more vulnerable with it's buffs stripped vs PLD or RUN who naturally have silly-strong defenses even when stripped or can recover their defenses very quickly.
PLUS
PLD can get stupid-high HP which is a factor on some NM's special attacks. I think RUN can as well? Not sure on that one actually. BLU can also get high-er HP but not without sacrifices I believe. I could be wrong.
***Edit***
More to it, I honestly dislike tanking myself because I have trust issues with WHMs that I don't know lol. Because if they are too slow on the cures/nas/etc I'm kinda screwed. I mean it's kinda hard to Magic Fruit when the NM is trying to pull off a combo.
I would MUCH prefer to sit there with an awesome tank that can actually hold hate against me so I can have some fun and really tear that poor NM to shreds! The best tanks I've ran with are usually able to hold that NM for around 35% of it's HP with me going as hard as I want before it eventually starts swinging at me. It works more favorably that way imo. BUT I am prepared to tank if its a slow night and one can't be found.