The song appears to be about the protagonist facing Satan during the Apocalypse. The protagonist is Satan's "chosen one," and seems paralyzed with fear. Along with "N.I.B.," this song added to the perception of the general public that the band members were devil worshippers (which the band has always denied).
According to the band, the song was inspired by an experience that Geezer Butler related to Ozzy Osbourne. In the days of Earth, Geezer painted his apartment matte black and placed several inverted crucifixes on the walls. Then, one day, Ozzy brought round a book about witchcraft, which Geezer became extremely fascinated by. One night, he read the book and placed the book on a shelf before going asleep. When he woke up, a large black figure stood in front of him, staring at Butler while he as he said, "crapped myself." After the figure disappeared, Butler ran to check out the book, only to find that the book was gone...He then told Ozzy, who wrote the lyrics to what would become Black Sabbath: "What is this that stands before me? Figure in black which points at me".
The main riff is constructed with a harmonic progression including an interval of tritone, also known as an augmented fourth. That interval was banned from medieval ecclesiastical singing because of its dissonant quality, which led monks to call it diabolus in musica—"the devil in music." Because of that original symbolic association, it came to be heard in Western cultural convention as "evil". Today the interval continues to suggest an "oppressive", "scary", or "evil" sound. Heavy metal has made extensive use of diabolus in musica because of these connotative qualities and this riff is one of the most famous example of its use in heavy metal. The "Black Sabbath" song was one of the earliest examples in heavy metal to make use of that interval.
This part of the song was sampled on Ice T's song Midnight on the OG: Original Gangster LP.
Black Sabbath is used as the opening track on both of the band's greatest hits (We Sold Our Soul for Rock 'n' Roll and Symptom of the Universe: The Original Black Sabbath 1970-1978) and is a regular installment of their live shows. A video for it exists and can be found on the Black Box DVD.
A version of this song from Black Sabbath's first demo exists on the Ozzy Osbourne compilation album The Ozzman Cometh. The song has an extra verse with additional vocals from Ozzy, right before the bridge into the fast part of the song. This song is a playable song in Rock Revolution.
In 1994, the band Type O Negative recorded a version of the song for the Black Sabbath tribute album Nativity in Black.