Dark Souls Fun Fact: Black Knights can see Invaders (DS3)
As far as I know, this is only a thing in Dark Souls 3, not Dark Souls 1. Dark Souls 2 doesn't have any Black Knights in it, so obviously not that one either.
I think it's a nice way to give the Black Knights the impression of higher intelligence. They are presented as a force to be reckoned with compared to normal enemies, so giving them this extra perception does a good job making them more threatening and interesting.