I think Sabbath gets the nod, because they tended to be more dark, than just signing bluesy songs about being cheated on. And so it seems to be “Heavy Metal” is to do songs about heavy subjects.
For example, that’s why I would have been hard-pressed to ever call Van Halen “Heavy Metal”, even though for sure EVH had a lot of metal influence in his playing, but I would just classify it as “hard rock”.
or “pop metal”...