Just as a heads up, black knights are supposed to be hard. Usually whenever you encounter them for the first time in an area, they are at least 5 times harder than anything around it. Typically they are guarding something worthwhile and drop some nice stuff as well.
Lure him back to the bonfire, climb the ladder. At this drop to the top of the stairs. When he's under u, drop on him with the plunging attack. Rinse and repeat. Or strafe behind him and perform the back stab move.