Intelligently, From Soft put no black knights in any of the chaos areas where there was demons or where they would end up near demons, as the AI in the game wouldn't be good enough to differentiate that the black knight should aggro on demons and demons onto the black knight.
So we only see black knights near other enemies in the game that are not demonic, whom they don't attack since they are not linked to the chaos flame.
Now then, the only enemy that we see them attacking, and thus know they attack other than the implied demons, is the player.
Why are they attacking a random undead? They are consistently near plenty of undead, such as in the burg, or Oscar, but never really bother to do anything. If aggroed and near other undead they only see the player as a threat.
It seems that the knights are hardwired to attack the player unlike any other undead.
This makes me believe that the player is in some way connected to the chaos flame, that the character you control is unique in that some way they are tied into both the chaos flame and the dark soul, unlike most undead that are only affected by the dark soul.
Just a thought.