Bjeorr
Go to the Castle Gate during Day and Evening to talk to the Twins for the Dark Knight Job. (The Job where you fire Ballistas at them for Diamonds)
During Mornings (i'm guessing the Twins have the late shift), there are other Guards there at the Gate, Renny and Stimper.
You can also Duel them via Library if you Rent a Room.
Dark Knights usually appear at the certain Jobs during Night, while cutting wood, mining etc
There are different types of Dark Knights depending on location as well, Top, Bottom etc