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Well, I guess my question becomes; Are you trying to make it so that the Blue Mage can learn powers from other PCs; in other words, is this a deliberate feature you are trying to add? If so, WHY?
The characteristic of the Blue Mage is learning monster/enemy unique powers, not merely miming another teammates abilities (a possible Paragon path perhaps, to recreate the Mime class from FFT); besides which, there is an inherent imbalance to such an ability which stripps the class of one of its internal limiting factors, non-scalable powers. While I can see the argument and even FF precedent for allowing a Blue Mage to learn
Magic Missle from a Drow opponent, allowing the player Blue Mage to learn the same from a party member has not such precedent, and will essentially make the BLue Mage an Uber class filled with every ability of every class ever used in a party, and all scaleable, as they would just re-learn the power when the party member levels again... Personally, I'd eliminate such a feature or ability altogether both for mechanical and flavor reasons.
A blue mage was only able to heal of his own blue magic only if he captured the ability to cure from a monster/opponent, not because there was another White Mage in the party. The blue mage is capable of being unparalleled in the possible versitility of their powers, and can potentially 'fill in' for fallen party members, but ONLY if they had exposure to the right mobs. By allowing them to learn from other party members,
you are assuring that this "possibility" is automatic. And again, there would be no incentive to learn a non-scaleable mob power when they could more readily learn effectively scalable powers, with no risk, from their buddies.
The ability to learn from other PCs or even non-hostile npcs does not fit the class concept, and is the only thing requiring you to introduce your "learn and forget" method of blue magic, another non-conceptual fit. Eliminate one, and you can safely eliminate the other, and bring the class back to concept.
So, I guess how I'd do it is, let them learn, effectively, as many powers as they can permanently.
This unparalleled 'diversity' is intrinsically limited by the difficulty in learning a worthy power, the non-scalable nature of the powers learned, and the actual availability of powers to be learned (which is set by the DM in the process of the campaign), and finally the lack of other internally learned class powers. Furthermore, what should be limited, like the Wizards of Old Editions, is not the number of options they have, but in how many different options they can utilize at a time. For this I'd simply say that, for example, a 1st level Blue Mage is limited to using a single Encounter Power during a particular encounter (just like everyone else, and ignoring racial bonus powers), even if they happen to know four such powers. Simply use the powers per level column already present for all classes to establish the baseline for the number of unique powers that the Blue Mage can manifest per grouping (at will, Daily, Encounter, and utility). This is not too disimilar to the spell book concept of old and new wizards, other than I'd suggest allowing the decisions to be made "on the fly", and resetting at each appropriate time phase (with At-wills being reset each encounter).
As implied in the preceeding, I'd eliminate all of the internal spells altogether; the Blue Mage has always been severely crippled in the absence of battle-won captured abilities. They should not receive automatic powers; that is one of the 'prices' paid for being in the class. I would change the capture powers to be "class features", like a Paladin's Lay on Hands or Cleric's "Divine Channelling" abilities, and make them available so many times per day based on either their Intelligence or Wisdom Modifiers, removing the current language regarding spending healing surges to extend the time they have access to the power. To reinsert the idea of expending healing surges, then you could allow, instead, the Blue Mage to exceed this limit by spending surges for additional attempts.
I would also add a minor (say +2) bonus to the learning rolls of particular groups of powers based on the 'tactics' of Blue Mage chosen; for instance giving the "Blue Warrior" character a +2 to their learning rolls for powers defined as "exploits" or otherwise obviously martial abilities, and give the bonus to Blue Wizards for learning Arcane or Divine spells. Just a momentary, on the spot, suggestion to help reinforce the distinction between the two 'types'.
Also, keep in mind that, since the powers being learned are not influenced by the Blue Mage's characteristics, this class is potentially less stat heavy than others, which is why I suggested the character's ability to learn powers is limited by a stat modifier, in addition to the actual learning roll itself. Perhaps, for balance's sake, use INT for the learning roll, but WIS for the number of times it can be attempted, both assignments of which are logical and consistent with their own definitions and use in the class.
Finally, I'd change the number of Healing Surges down to 6+ (especially since they no longer fuel the learning ability directly) which is more consistent with the Rogue and Warlock classes. Otherwise, the proficiencies and other abilities seem to be right on.
So, that is what I'd do with it, in a nutcase...er... shell
