Quote Originally Posted by Wyssahtyn View Post
"'But how!?' I hear you cry. It's simple! You have to attune your mind to the instincts of the beast! When a creature is driven to attack, you can see into its soul. When you overcome it in battle, you absorb some of its aether and make its power your own!" - Martyn going over the mechanics of how Blue Mages learn their abilities.

Appears that you do need to kill the enemy in question to absorb its aether, similar to SMNs and primals, I suppose. I'd chalk up learning stuff from Azulmagia a result of gameplay personally.
Hildibrand stuff isn't always the most easy to place in terms of canonicity, but Ultros and Typhon also have BLU spells you can learn, and they definitely don't die in that fight. You also can't really "kill" the robots for Drill Cannons and the like-- they're not alive in the first place, you're just breaking them (and presumably there's nothing stopping a talented mechanic from fixing them). I'd say that using the word "overcome" instead of outright saying "kill" makes it kind of ambiguous as to whether or not killing is strictly necessary; it might still qualify as overcoming your opponent if you hit them with a stick until they were unable to fight any further, for example. Could be way off, though!