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Ok, thanks for the clarification.
However, I still think that any power should be at least Moderate difficulty; as it is, you could have the Blue Mage learn an At-Will power from a critter of a fairly higher level than themselves.
EG: a 1st level Blue Mage with a 16 Int would still only need to roll a 12 to learn an At-Will power from a 10th level mob. (5+3+12 = 20 = 15+5 DC). That is simply too easy. This means learning attacks that are +14 or +15 to hit, at first level, as At Will powers.
Since, at current, this ability allows them to learn any ability used, this means that the surviving blue mage (Hide and Run!) has access to a substantial attacking or spell power that no other character would be able to reach for some time. While this can be controlled by an attentive DM, I (for one) do throw PL+4 or more level creatures at a party for the big battles, which even then would provide more of a boost in power than I'd like, or that fits the Blue Mage's inherent weaknesses, that of using fairly weak (comparatively) monster abilities. Furthermore, if you allow the learning from PCs or non-threatening NPCs, what is to stop them from Hiring a 10th level wizard or cleric to fire off At-Wills until they learn them all (at a tenth level of ability)?
Yes, I realize that "role playing" should be a consideration in this, but it is painfully clear from the direction 4E went with things, that RP and Flavor are no longer used as limiting factors in balancing classes.
Anyway, the difficulty should, imho, go up by +5, and you might want to consider a level cap to what a Blue Mage can actually learn from a mob.