UPDark Knights exist specifically to combat the corruption on Ishgard. Someone who uses that style of combat but isn't indoctrinated to that cause isn't a Dark Knight, they're just someone who uses that style of combat. It's a specific title referring to a role/career, not a catch all for people who use greatswords and emotional aether.
Edit: less Batman, more Punisher.
Edit: less Batman, more Punisher.