I question how early "black metal" was being used, and what it was specifically categorizing if it was being used. It certainly didn't have any widespread usage that I remember before the early 90's. I see "Christian metal" as less an issue of coining a term than it is of just describing what is being done, thus it is going to be hard to pin down the term. Since the late 60's Christians were making rock and roll, and likely calling it "Christian rock" in at least some circles, though the coined term was "Jesus Music". The earliest band that I might call Christian metal is probably a band called Stronghold which released a Sabbathesque album called Fortress Rock in about 1981. There were others around (Ressurection band, Jerusalem, Barnabas), but they seemed to be more like hard rock to me. On the other hand, "White metal" is more of a term to be coined, and always carried the connotation of something assigned by "outsiders".