Dark knights, also known as black knights, were a minority group of knights who refused to serve lords or kings. Instead they either looted and raided, hunted bounties, or lived off the charity from the more humble folk, in much the same manner as a Japanese ronin. As they had no master, and thus no squire, they would paint their armor a signature black color to prevent rust and damage. Dark knights were considered by many a necessary evil, as they would protect villages and lesser people from invaders and bandits, but would also cause problems in the regions they dwelt in to sustain themselves. In some legends, black knights were reputed to be nigh-invincible in combat.
See Special:Whatlinkshere/Etymology:Dark knight for a list of articles using this term.