Heavy metal music
Critics disagree over who can be thought of as the first heavy metal band. Most credit either Led Zeppelin or Black Sabbath, with American commentators tending to favour Led Zeppelin and British commentators tending to favour Black Sabbath, though many give equal credit to both. A few commentators—mainly American—argue for other groups including Iron Butterfly, Steppenwolf or Blue Cheer. Deep Purple, the third band in what is sometimes considered the "unholy trinity" of heavy metal (Black Sabbath, Led Zeppelin, and Deep Purple), despite being slightly older than Black Sabbath and Led Zeppelin, fluctuated between many rock styles until late 1969 when they took a heavy metal direction.
Murder Music: A History of Black Metal
The documentary links the origins of black metal to the birth of heavy metal itself with the formation of Black Sabbath, known for its early horror-inspired lyrics, it is even said that bassist Geezer Butler, who wrote many of the band's lyrics is to be considered the "godfather of black metal". It quickly moves to the existence of a similarly-named band, which was also often confused for Sabbath, called Black Widow.