As someone who is a huge fan of all hard rock, "heavy," heavy metal", "grunge" and raucous punk rock, the lines/categories get blurred.
Wikipedia does a decent job of breaking down what heavy metal is and the origins. Some say the Beatles' song Helter Skelter was the first heavy metal song. I would not agree.
As with all things related to music and art, opinions will vary.
I agree with you that not all Black Sabbath is necessarily "heavy metal."
Wikipedia's take:
"With roots in
blues rock,
psychedelic rock and
acid rock, heavy metal bands developed a thick, monumental sound characterized by
distorted guitars, extended
guitar solos, emphatic
beats and
loudness.
In 1968, three of the genre's most famous pioneers – British bands
Led Zeppelin,
Black Sabbath and
Deep Purple – were founded."
"The first use of "heavy metal" in a song lyric is in reference to a motorcycle in the
Steppenwolf song "
Born to Be Wild", also released that year:
[94] "I like smoke and lightning / Heavy metal thunder / Racin' with the wind / And the feelin' that I'm under"."
An early documented use of the phrase in
rock criticism appears in Sandy Pearlman's February 1967
Crawdaddy review of
the Rolling Stones'
Got Live If You Want It (1966), albeit as a description of the sound rather than as a genre: "On this album the Stones go metal. Technology is in the saddle—as an ideal and as a method."
[95][nb 1] Another appears in the 11 May 1968 issue of
Rolling Stone, in which
Barry Gifford wrote about the album
A Long Time Comin' by U.S. band
Electric Flag: "Nobody who's been listening to
Mike Bloomfield—either talking or playing—in the last few years could have expected this. This is the new soul music, the synthesis of white blues and heavy metal rock."
[97] In the 7 September 1968 edition of the
Seattle Daily Times, reviewer Susan Schwartz wrote that
the Jimi Hendrix Experience "has a heavy-metals blues sound".
