Good bless them if they make Guitar Hero: Black Metal!
I've been saying for years that, by isolating their key and initial demographic of rockers and metalheads, the GH/RB franchises fail to establish any proper grounding for further games.
While expansions into Pop Music and Country can and have made for good short-term gains and introduction of the series to younger and more mainstream audiences, this draw as a sales tool works only as long as it functions as a fad: a product, person, or activity that enjoys a short, but grossly successly run of popularity in modern culture. Frankly put: Rockers aren't just there for the bands, but for the culture and lifestyle it represents. It's a chance to vicariously life the rock and roll lifestyle they so crave, craving not only the glitz, but the hard times as well (theres a certain romance in that). I've written sociology papers about the early connections of heavy metal/hard rock with the development of video games (especially in the development of modern console gaming (circa early 1990s), and there is a great deal of overlap in rocker/gaming culture.
Maybe a foray into Black Metal isn't the best next step for these companies, but if traditional die-hard hard rock/metal fans are acknowledged and respected more in upcoming titles, I can see a return of good percentage (for some the damage has already been done) of die-hard fans to float future iterations of the title(s).