Thereotically, music should be the most important aspect when a music genre is the subject at hand.
Obviously, but I reiterate, Black Metal is not just a music genre, and should not be considered as such in any but the most basic and primitive discussions of the musical components. As thisisaformicatable has said, applying the same standards of classification for all genres does not adhere to reality, and the reality of Black Metal is that the conceptual framework is inseparable from the craft.
I wasn't holding music to a lesser standard than art or literature. The symbolism displayed by music is much different than art or literature because you hear it rather than see it. The visual component is irrelevant because, technically, it's not part of the music but rather a prop that compliments it.
Once again, you're viewing the subject as only the musical component when the artwork very often is an integral conceptual piece with the work as a whole. The artwork is not merely a "prop," but an essential piece of the puzzle. Of course this is not universally true, but largely it is.
It is completely out of place. I agree. However, if the band still manages to convey a dark atmosphere by applying the characteristics of black metal, than it's still black metal.
Aesthetically, on the basest of levels, but this is not the classification being discussed. Musical similarities do not dictate co-relations.