Addressing too many Useless Powers
Cariborne claimed nearly every skill is useful. For Blue Mage, it would be similar. That is to say, the % of useful skills for Blue Mage would be similar to that of other classes.
m3eansean stated you can't give a Blue Mage more abilities than other classes. I agree. The level 50 Blue Mage has exactly 28 abilities. The key difference is they have more dormant abilities that can be swapped out, but also less automatic (quest free) abilities.
Even if you are in a dungeon, a DPS can use any ability in their tree, by going to the menu, even if it's not on the hotbar. When crafting, a crafter is stuck with whatever slots they chose, and they can lose their mats by canceling half way and starting over with a new action, but also sacrificing another good action.
Meanwhile, the Arcanist Class. has @99 actions, counting Summoner, Scholar, and Cross class. PVP puts them at over 100. Once locked into place and entering a Quest, they are stuck with a core list of actions.
Blue Mage Enemy Actions over time...
It seems logical that if the blue mage action is copying the enemy's stats that the relevance of some powers would wane as they "grew out of" them. I kind of like this feature for some powers because in addition to being a low level power, it should also retain a low level MP cost. This is particularly useful when you run low on MP. Sure, the power does 12 damage, even at level 38 or even 88, but it only costs 4 MP.
About Control for Blue Mage
(Control is your approximation of doing that, if control is < enemy healing/attacking potency, then your spell costs more MP than normal and/or has a % chance of fail, if Control is equal or Higher, then the MP cost is normal and % chance to fail is 0. "Mastery" comes from repeated use of a skill; it lowers the % chance of fail, but doesn't lower the MP inflation, only increasing Control will do that)
Blue Mage: pure Class or Double Job?
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.