The most common blue mage styles are spell caster and gun mage. There is a gun class coming in Heavens Ward which would make a good DPS "secondary" class for Blue Mage, allowing for a "Gun Mage" to use "Blue Bullet". Such a Class would definitely be a DPS.

A High Vitality isn't unknown in Blue Mage History. A handful of Enemy Traits or Powers can make them quite tough. Particularly, the monsters where you have to use a hp_down type potion/poison to actually have a solid chance, despite their level being similar to yours. These types of powers, as well as Shrinking powers, and damage down mixes would allow a blue mage to collectively tank quite well, though somewhat non traditionally.

I suggest the Blue Mage's best defense features be Magic Defense, Magical Defenses, and Vitality Boosters, if we are talking about a Tank base.