While I was trying to put the story together I ran across the Wiki page which was far and away better than what I was typing. So not needing to reinvent the wheel, here is what they have to say:
According to the band, the song was inspired by an experience that Geezer Butler had in the days of
Earth. Butler, obsessed with the occult at the time, painted his apartment matte black and placed several inverted
crucifixes and pictures of
Satan on the walls.
Ozzy Osbourne gave Butler a black occult book, written in Latin and decorated with numerous pictures of Satan. Butler read the book and then placed it on a shelf beside his bed before going to sleep. When he woke up, he claims he saw a large black figure standing at the end of his bed, staring at him. The figure vanished and Butler ran to the shelf where he had placed the book earlier, but the book was gone. Butler related this story to Osbourne, who then wrote the lyrics to the song based on Butler's experience.
[4] The song starts with the lyrics:
A version of this song from Black Sabbath's first demo exists on the Ozzy Osbourne compilation album
The Ozzman Cometh.
[5] The song has an extra verse with additional vocals before the bridge.
[6] It's one of the band's most frequently performed tracks, being featured on every single tour of their career.
The riff is fairly simple, highlighting the dissonant and dark sound of the tritone against a stagnant harmonic rhythm. <a href="
https://en.wikipedia.org/wiki/Black_Sabbath_(song)#cite_note-8"><span>[</span>8<span>]</span></a> This particular interval, the tritone, is often known as the
diabolus in musica,<a href="
https://en.wikipedia.org/wiki/Black_Sabbath_(song)#cite_note-Wolf-9"><span>[</span>9<span>]</span></a> for it has musical qualities which are often used to suggest
Satanic connotations in Western music.<a href="
https://en.wikipedia.org/wiki/Black_Sabbath_(song)#cite_note-Wolf-9"><span>[</span>9<span>]</span></a><a href="
https://en.wikipedia.org/wiki/Black_Sabbath_(song)#cite_note-10"><span>[</span>10<span>]</span></a><a href="
https://en.wikipedia.org/wiki/Black_Sabbath_(song)#cite_note-11"><span>[</span>11<span>]</span></a> The song "Black Sabbath" was one of the earliest examples in heavy metal to make use of this interval,<a href="
https://en.wikipedia.org/wiki/Black_Sabbath_(song)#cite_note-Wolf-9"><span>[</span>9<span>]</span></a> and since then, the genre has made extensive use of
diabolus in musica.<a href="
https://en.wikipedia.org/wiki/Black_Sabbath_(song)#cite_note-Wolf-9"><span>[</span>9<span>]</span></a><a href="
https://en.wikipedia.org/wiki/Black_Sabbath_(song)#cite_note-12"><span>[</span>12<span>]</span></a>
The riff was created when bassist Geezer Butler began playing a fragment of "Mars" from
Gustav Holst's The Planets suite. Inspired, guitarist Tony Iommi returned the next day with the famously dark tritone.