this has been around since beta... as in the start of it.
I don't think they ever said what it was exactly, but my guess would be some occluders on their maps not filtering light properly from the sources, or possibly a left over 'transparent' (and immaterial) texture...maybe a wall or something from testing that was left over.
I ran into it really bad in k7 during beta... one side of the floor and hallway was black... when you ran past the black textures, you could turn around and see out. Then further down the hall, the textures would shimmer and darken like your shots...
I think they said something about testing the maps and textures to see what was up, but somehow I don't think they ever totally fixed their rendering engine.