The Garleans have been using ceruleum as fuel for a long time; it wasn't until Solus (Emet-Selch) introduced them to it that magitek became the cornerstone of their arsenal.

Gunbreaker gunblades are more evenly balanced, with both offensive and defensive capabilities. As one would expect from a tank class taking cues from VIII's Squall, GNB is the most offensively oriented tank class in the game to the point it's the only class with a(n up to) 6-hit combo via Gnashing Fang / Continuation. That does come at the cost of defensive options compared to other tank classes, but GNB still has plenty of those for themselves, other single targets, or the whole party.

Garlean gunblades are purely offensive, but it's debatable whether that's due to their design or the fact they aren't used by playable characters.

Regardless, Garlean magitek is all fueled by ceruleum; if it generates blue flames, it's probably ceruleum-derived. (Memoria Misera Varis does produce other colors, but that's not real anyway and the predominant color is still ceruleum blue.)

Zenos did use a crystal to force himself to manipulate aether in order to defeat (kill) his instructor, but it's unclear whether her continued doing so afterwards. That was the point behind Aulus' experimentation, and he has enchanted weapons in every battle to compensate for this "deficiency." However the dark aura is almost explicitly tied to Ame-no-Habakiri; it doesn't show up until Zenos pulls the aforementioned weapon out. As for his revolver scabbard, that bit was a touch of realism - katana (Zenos' adopted weapon of choice) dull very quickly, so the revolving scabbard lets him switch to a fresh blade to stay on the battlefield longer. (Considering this design choice, if anything I assumed the revolver scabbard has built-in sharpeners so while one katana is in use the others are being repaired, but considering he now exclusively wields magical weapons I'm not sure it's necessary anymore.)