Dark Knights are treated as pariahs due to the source and nature of their powers, as well as what they use them for. The Abyssal magic wielded by DRKs makes them scary, and their willingness to cross legal boundaries to make sure justice is served puts them at odds with the state. The common folk saved by one such Dark Knight are undoubtedly appreciative, but it's a path that chooses you - no one should want to become a Dark Knight, as it involves fully accepting not just the physical but mental and emotional toll living by the sword takes on you.

Common folk who witness a Dark Knight in action and aren't in need of saving tend to be terrified of them thanks to their brutal fighting style and Abyssal powers, though. The nobles eavesdropped on before finding Fray's corpse were scared witless of him before he fell.

(For the record, I always read the class as something like Tales of Vesperia's Yuri Lowell, but with more armor and dark powers.)

Black Mage? Never touched the stuff... ask someone else...