This kind of thing upsets me. I always felt like one of the important things to black metal was depth, and one of the ways to achieve it was through obscurity. The listener is forced to pay attention and not shrug off the music as mere noise - and suddenly, subtle details laid by the artist begin to shine through. Swimming through the waves of sound also begins to achieve significance in itself.
Lyrics should be no exception. They need not be uninterpretable, but to make their significance stand out at face-value seems contrary to writing black metal.