Hope this helps.
Allmusic.com reviews of the first three albums:
Black Sabbath - s/t
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.
Black Sabbath - Paranoid
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). The subject matter is unrelentingly, obsessively dark, covering both supernatural/sci-fi horrors and the real-life traumas of death, war, nuclear annihilation, mental illness, drug hallucinations, and narcotic abuse. Yet Sabbath makes it totally convincing, thanks to the crawling, muddled bleakness and bad-trip depression evoked so frighteningly well by their music. Even the qualities that made critics deplore the album (and the group) for years increase the overall effect the technical simplicity of Ozzy Osbourne's vocals and Tony Iommi's lead guitar vocabulary; the spots when the lyrics sink into melodrama or awkwardness; the lack of subtlety and the infrequent dynamic contrast. Everything adds up to more than the sum of its parts, as though the anxieties behind the music simply demanded that the band achieve catharsis by steamrolling everything in its path, including its own limitations. Monolithic and primally powerful, Paranoid defined the sound and style of heavy metal more than any other record in rock history.
Black Sabbath - Master of Reality
With Paranoid, Black Sabbath perfected the formula for their lumbering heavy metal. On its follow-up, Master of Reality, the group merely repeated the formula, setting the stage for a career of recycling the same sounds and riffs. But on Master of Reality Sabbath still were fresh and had a seemingly endless supply of crushingly heavy riffs to bludgeon their audiences into sweet, willing oblivion. If the album is a showcase for anyone, it is Tony Iommi, who keeps the album afloat with a series of slow, loud riffs, the best of which "Sweet Leaf" and "Children of the Grave" among them rank among his finest playing. Taken in tandem with the more consistent Paranoid, Master of Reality forms the core of Sabbath's canon. There are a few stray necessary tracks scattered throughout the group's other early-'70s albums, but Master of Reality is the last time they delivered a consistent album and its influence can be heard throughout the generations of heavy metal bands that followed.