Has anybody actually figured out who this "White Knight" is?

That would be ganguro (ガングロ), which IIRC means "burnt black" -- it's a fashion involving tanned skin, bleached hair, and loud makeup. From what I've read, the ganguro trend goes against the "normal" ideal beauty of pale skin, dark hair, and not loud makeup. Japan is fairly homogeneous with ethnicity, and there's not much history of black people there, so "blackface" most likely wasn't even considered when that trend was set.

But like you mentioned, it's very unlikely the average American would know that then, much less that lolcow.