Unfortunately, the Blue Mage is an incredibly difficult concept to pull off in d20 (and not much easier in other games). In d20, the Blue Mage either learns Sp or Su abilities. If the former, it's not terribly interesting. If the latter, it has the potential to become monstrously broken very fast. The DM has to call all abilities fair game (arguably broken, and untrue to the FF flavor) or set an arbitrary limit (which, being arbitrary, is liable to cause upset and resentment). Also, it's basically impossible to measure the Blue Mage's power against the CR system, since he may be vastly stronger or weaker depending on the number of Blue Magic-learnable enemies the DM uses.
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As a very rough d20 conversion, I'd make the Blue Mage identical to the sorcerer, but instead of learning ANY new spells after first level, he has to have a spell on the sorcerer spell list cast on him, which he can then learn.