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The Yardbirds- Stroll On (1966) and Blue Cheer - Summertime Blues (1968) already had some heavy riffage in the rhythm section.
But they still had some bluesy fillers.

Punk first came out with this idea of killing all the bluesy flourishing shit in rock. So they did mostly boneheaded riffage with some occasional Chuck Berry-style licks.

Basically what Sabbath did was slowed down punk. They took the heavy riffage part but played it at downtempo speed with ominous doomy vocals.