What's really interesting is that the way one goes about learning Enemy Skills is based on the same principle that Summoners use to be able to create Egi. Namely, when something dies its aether gets released into the surrounding environment, including the person who killed it. It's possible to access that "foreign" aether and do things with it. In the Blue Mage's case, that "foreign" aether is used as a template to recreate an ability of the dead creature using external aether. A Summoner goes a lot further with the technique and uses the "foreign" aether to not just recreate an ability, but to recreate a miniature version of the creature itself with their own aether. Summoner even goes so far that they can make their own aether look like the aether of a creature they killed and cast their spells themselves.

This also plays into what makes the two jobs different. Blue Mage can mimic the attacks of a lot of different, common monsters without much trouble it seems, but it uses the aether of the environment to do so. A Summoner only uses their own internal aether (as far as we know) but they can only make miniature creatures based on primals, which are entirely made of aether and are really ideas given life. While it is technically possible to make miniature versions of anything, the math/geometries of creatures that are based in the material realm is impossible to calculate without an Allagan supercomputer. While they can mimic a creature's moves, it requires a lot of build-up time and so far, it looks like a Summoner can only do that with one creature.

So yeah, both Blue Mage and Summoner are both based off of learning things from the aether-traces left behind in someone when something is killed, but Blue Mage focuses on breadth using external aether while Summoner focues on depth using internal aether.