Regurgitating what an anon said in one of these threads before:
"Blue mages are appealing to players that want to be given a new toolset during combat that they optimize on the fly. Deckbuilder roguelite. The enemies you encounter each represent "gain a function" and the player assesses the boardstate to assemble the best sequence of actions over several turns. Multiple lines of play need to be present, and those lines can and should branch, and different non-numeric modifiers should be present that muddy up the EV of different lines. This means that each enemy should have multiple abilities that the blue mage can steal, stolen abilities should have a limited duration, and permanent bonuses to the character should expand rather than limit the number of lines available to them so that they scale in complexity as they level up, same as any other class. That means that saving abilities longterm is retarded because it goes against the basic design and appeal of the class. Character progression needs to be similar to sorcerer metamagic, a function modifier."