I doubt this will ever happen. Before "dark music" as a genre, stuff like Country, Metal or Drum n Bass would get priority. "Dark" music isn't really a genre anyways.. (though people will probably argue Anime as a genre isn't any better)
As others have said, there is not a real genre for "dark". You'd be better off searching other individual terms, such as metal, death, etc. You'd also be able to find "dark" in tags, I'd assume?