AC/DC used to be the heaviest, darkest, blackest metal band in the world. Then Metallica happened, and now people call it roots rock. AC/DC never considered themselves heavy metal, even when their fans did. I personally think of them as heavy metal, but that's because I refuse to change, and also because I refused to surrender the category to hair metal. Led Zeppelin never thought of themselves as heavy metal. Their fans never thought of them as heavy metal. Metal fans never thought of them as metal. Critics applied the word to some of what they were doing, but I guess you can't be part-time metal, so it didn't stick. I also consider Sabbath to be the original, archetypal heavy metal band. BiTD, I never heard anyone describe them otherwise. I think they probably also thought of themselves as such.