It's difficult to overstate the importance of this song to both Black Sabbath and heavy metal in general. The band was inspired to write the song when they saw a movie theater playing the 1963 Boris Karloff film Black Sabbath. Bassist Geezer Butler observed that people loved seeing scary movies, but there wasn't much scary music out there. This was also the peak of the hippie era and the four working-class kids from Birmingham, England were sick of songs about peace and love. The group wrote a song called "Black Sabbath" and changed their name from Earth to the same.
The track is an absolute masterpiece and does feel like a mini audio-horror movie. There was nothing like it on the charts, and the group quickly began writing similar songs. Five decades later, "Black Sabbath" remains a high point of their live show.