A lot of heavy metal is the blues played very fast and with heavy distortion. So many of the solos by Pantera use the blues scale

Hard rock and heavy metal have their roots in the blues. All of the early bands that defined those genres straddled the line between blues and rock. Led Zeppelin were heavily influenced by the blues; but also if you listen to a band like Deep Purple you will hear the blues influence all over the place. Hendrix, of course, supercharged the blues and it would be difficult to say if he was a blues or rock performer. Even Black Sabbath showed their blues influences in many songs.

Half their songs are covers of older blues songs.

Plagiarized.

They really did claim to have written "Killing Floor" with a stupid title of "The Lemon Song." Howlin' Wolf had been to london and recorded with Clapton. Hendrix had played a scorching version of it on BBC Radio One. No way they could have missed just that one. Then all the Willie Dixon. It's embarrassing how often Page/Plant claimed to have written classic blues and robbed working folk artists. Lowered my opinion of them significantly to see it so if you want to hero-worship them maybe skip compilations like this:

https://www.youtube.com/watch?v=tiiY4ciKFQA

And that's just some of it. When they stopped doing it the quality of their output fell off a cliff.