Yes, but culture changes because it's partially genetic and not completely genetic. Understand? There are constants and universals in human culture. You might want to check out Donald E. Brown's list of human universals, for example.
The logic here is quite simple. Family members behave in more similar ways to one another because they're genetically more alike to each other than they are to non-family members. Likewise, individuals of the same ethnic group will behave more like each other than to individuals of other ethnic groups because of greater genetic similarity between members of the same ethnic group compared to individuals of different ethnic groups.
I don't know, every time I either go to a metal gig or see footage of metal gigs on the net/TV I just see a sea of white faces. I've never even met a non-white metal fan, to be honest. Not that they don't exist or anything but they're certainly a rarity, so to speak.
As for the idea that heavy metal is somehow the offshoot of a black invention, well, that's just plain wrong in my opinion. Metal comes from rock which mostly comes from blues, gospel, folk and country. The folk music in question was Scots-Irish folk music played by immigrants who came to America from Scotland and Ireland. Blues and country, were themselves originally based on this Scots-Irish folk music, though blues included much traditional African influence. Gospel, also had heavy African influences, despite originally being based on the religious music of the Scottish and Irish immigrants. So, blues and gospel were just Africanised versions of traditional Celtic music really. Now, I don't know exactly what metal you listen to, but I listen to death/black/folk metal, and it's mostly these styles of metal that have no black elements or influences left in them. Metal musicians have, without intending to, completely removed black influences from their music, on the one hand and added many aspects of European classical and folk music on the other. Indeed, the Jewish-British sociology scholar Keith Kahn-Harris noted that one of the most striking facts about extreme metal is its complete lack of influence from black music. You have to ask the question, then, why did white musicians take music of bi-racial origins and unwittingly remove all of the black elements?