I personally would use 4E and I would make it a multiclass option. The initial multiclass feat would grant a variable bonus which the PC can change every time he (or his party) defeats a monster. The specific bonus would depend on the general type of monster defeated: a soldier would provide a bonus to AC, a brute would provide a bonus to melee damage rolls, a skirmisher would provide a bonus to initiative checks or movement, artillery would provide a bonus to ranged damage rolls, a controller would provide a bonus to damage rolls for bursts and blasts, and a lurker would provide a bonus to Stealth checks, or possibly a bonus to damage when attacking with combat advantage. Each time the PC defeats another monster, he can choose to retain his previous bonus or change it to the bonus granted by the newly defeated monster.
The various power swap feats can then be used to gain powers from defeated monsters. The DM should excercise caution and approve each swap (or he may decide in advance which powers are swappable), but as a general rule of thumb, I would say that the monster's at-will and recharge powers should become encounter powers, and the monster's encounter powers should become daily powers, of a level equal to or higher than the level of the monster. If the monster has special movement or utility abilities, they can become encounter or daily utility powers using the same guidelines. Hence, at-will or recharge attack powers from monsters of 4 to 7 level can become 7th-level encounter attack powers, subject to DM approval, of course.
There might need to be some adjustment to ensure that the powers scale with level, or this might be left alone to create an incentive for the blue mage to continually exchange older powers (with lower attack and damage values) for powers from the higher-level monsters that he defeats.
The advantage to this approach is that you get the flavor of the blue mage while keeping it mostly balanced. A blue mage character would have at most one encounter attack power, one daily attack power and one utility power from the monsters he defeats, with the rest of his powers coming from his base class. Even if he gets particularly weak or powerful abilities from the monsters, they are only a fraction of his overall capability.
It would be possible to take this idea further and come up with a paragon path and/or an epic destiny for character who want a higher proportion of their powers to come from the monsters they defeat. However, that would take a lot more work than I intend to put into this post.
