from allmusic: Black Sabbath's debut album is given over to lengthy songs and suite-like pieces where individual songs blur together and riffs pound away one after another, frequently under extended jams. There isn't much variety in tempo, mood, or the band's simple, blues-derived musical vocabulary, but that's not the point; Sabbath's slowed-down, murky guitar rock bludgeons the listener in an almost hallucinatory fashion, reveling in its own dazed, druggy state of consciousness. Songs like the apocalyptic title track, "N.I.B.," and "The Wizard" make their obsessions with evil and black magic seem like more than just stereotypical heavy metal posturing because of the dim, suffocating musical atmosphere the band frames them in. This blueprint would be refined and occasionally elaborated upon over the band's next few albums, but there are plenty of metal classics already here.
Paranoid was not only Black Sabbath’s most popular record (it was a number one smash in the U.K., and “Paranoid” and “Iron Man” both scraped the U.S. charts despite virtually nonexistent radio play), it also stands as one of the greatest and most influential heavy metal albums of all time. Paranoid refined Black Sabbath’s signature sound — crushingly loud, minor-key dirges loosely based on heavy blues-rock — and applied it to a newly consistent set of songs with utterly memorable riffs, most of which now rank as all-time metal classics. Where the extended, multi-sectioned songs on the debut sometimes felt like aimless jams, their counterparts on Paranoid have been given focus and direction, lending an epic drama to now-standards like “War Pigs” and “Iron Man” (which sports one of the most immediately identifiable riffs in metal history).