If we do get BLU, it isn't gonna be a tank, I can tell you that much. Some examples:

1. Blue Mages need to learn their spells from enemies (learning and tanking at the same time would be annoying and would be even worse for newbies who aren't skilled enough).
2. Blue Mages aren't good in heavy equipment (and even Yoshi-P can't make them look good in armor as it wouldn't even called "mage" anymore)
3. Blue Mages uses swords and maybe shields (if going by FFXI). We have a sword and shield job...it's called Paladin.
4. Judging from the HP pool of the other 3 tanks, BLU isn't gonna have enough to help it survive tanking a huge group of mobs even with VIT melds.
5. Blue Mages rely on INT and if memory serves "maybe" STR, so VIT would be a wasted stat for it compared to Paladin.

6. And this is imho, Blue Mage just doesn't meet the qualifications of being a tanking job. It's more valuable as a DPS (or in this case, a melee caster DPS). DRK imo is just as bad as a tank, and imo it should have been a dps, but they had no other original ideas for a new kind of tank for HW, so they figured: "Hey! Lets make this old dps job into a tank! What could possibly go wrong? Ha Ha!"

In short, BLU is a pure melee caster hybrid dps who can learn ememy skills/spells. It's a mage, and as such, it's not fit for tanking. Especially with it being...well...a CASTER.

I rest my case. GG.