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.