When I worked as a mailman, all my younger colleagues were huge metal fans, but only into current metal. I was pretty surprised to learn they had absolutely no love for Black Sabbath. None at all. The shame.
I'm no metalologist, but the riff at 3:15 in Sabbath Bloody Sabbath is the earliest example I can recall hearing of a muted-strings-metal-riff, the kind you'd hear all the time in newer metal. May be completely false, awesome riffage anyhow.