I never heard of the darkside of music. Maybe people would say modernistic or atonality is the darkside, because they don't like it, but if you liok at how it is composed it is really breathtaking. They composed songs on mathematics and so much different techniques that I would say it is not the darkside; but it is true that the feelings are gone, so maybe then it is the darkside.

Others would maybe say that nationalistic music like Wagners is dark, but I think music itself has no darkside except expressing darkness.