We are talking about heavy metal, not contemporary rock music in which Led Zeppelin may have had a hand in creating. But the harder, edgier version of rock, which is heavy metal, belongs to Black Sabbath.
Can't agree with that
Sabbath's 1st album wasn't until 1970, After Led Zeppelin had 3.
Communication Breakdown, Dazed and Confused, Whole Lotta Love, Heartbreaker, the Immigrant Song (and probably a few others) can be considered Heavy Metal and preceded Sabbath.