I too think that BLU would be most interesting as a tank class. As for a weapon, I feel that maces and hammers would be most interesting, as we basically have most iterations of swords already implemented, and it gives monk a buddy to share blunt damage with. As we once had a BLU who wields forks, the kind of weapon shouldn't matter much.

In order to make it tanky but not seem too much like a beefcake such as the existing tanks, I think a system where your MP bar functions as a "shield HP Bar" just like damage MP would be cool. You'd have to maintain your MP to maintain your mitigation like how BLMs regen their MP for consistent dps, albeit through a different mechanic. If that would seem to broken (as healers need to heal HP) it could be mitigation based rather than full damage substitution based.

(Also, as a tank the job icon would be blue. (>_o)b Very important.)