BY JIM DeROGATIS POP MUSIC CRITIC

Hardcore fans know that no band did more to define the nascent genre of heavy metal than Black Sabbath, and they invariably cite the group's first four albums (1970's self-titled debut and "Paranoid," 1971's "Master of Reality" and 1972's "Volume 4") as the cornerstones on which all of the noise that followed was built.

But the quartet continued to refine its brand of sturm und drang throughout its first incarnation, which ended when original vocalist Ozzy Osbourne left in fall 1977. Often overlooked in discussions of the black-clad maestros' masterworks is the 1973 epic "Sabbath Bloody Sabbath" (Warner Bros.), whose title track was to Sabbath what "Stairway to Heaven" was to Led Zeppelin--a crowning achievement that crystallized the group's sound, and which stands as a paradigm of everything that made the band great.

Originally a psychedelic blues combo called Polka Tulk (later Earth), the four mates from industrial Birmingham, England, changed their name to Black Sabbath in 1969, drawing inspiration from the title of a B-movie starring Boris Karloff (originally released in Italy as "I tre volti della paura" [1963]). Pairing a hefty dose of horror-movie imagery with the solemn minor-key riffing, deep bass rumble, wailing vocals and massive, plodding rhythms, they would go on to define the heavy-metal sound on classic tunes such as "Iron Man," "Hand of Doom" and "Children of the Grave."

The group partied hard and toured constantly through its early years, and by the mid-'70s, the pace was starting to take its toll. In 1974, drummer Bill Ward suffered from hepatitis and his first heart attack, bassist Geezer Butler was downed by kidney stones, guitarist Tony Iommi found himself on the brink of exhaustion, and Osbourne was deep in the throes of alcoholism and drug addiction.

In their late 20s, the musicians were finally starting to sense their own mortality when they retired to a castle in Wales to record their fifth album. The disc finds Sabbath broadening its musical palette considerably, incorporating synthesizers (played by guest keyboardist Rick Wakeman, a veteran of Yes), strings and acoustic guitars in addition to Iommi's trademark fuzz-driven Gibson SG.

An unusually tranquil and idyllic vibe permeates songs such as "Looking for Today," "Sabbra Cadabra" (a love song that ends with a demonically echoed laugh) and "Spiral Architect"--three tunes that are as close to pop ditties as Black Sabbath ever got. The pretty but sarcastically titled "Fluff" stands as unique in the band's catalog (it's a spartan acoustic guitar and piano interlude) and the dark, synth-dominated "Who Are You" marks Osbourne's first songwriting contribution to a Sabbath album.

This new, dare I say "sunnier" outlook was no doubt partly the inevitable result of aging--even the most miserable, angst-ridden teen can't help but mellow a bit by age 28. But it can also be viewed as evidence of the musicians having grown confident enough to begin tinkering with the sonic formula that they'd painstakingly perfected over the previous four recordings.

With "Sabotage" and "Technical Ecstasy," the discs that followed, some fans say that the band went too far in diluting Sabbath's infamous black magic. But "Sabbath Bloody Sabbath" strikes the perfect balance between experimentation and that familiar crunch. In retrospect, the liberal use of keyboards and softer melodic textures predict the "symphonic" or "black metal" sound of current underground heroes such as Cradle of Filth. And on the tougher songs, Sabbath never rocked harder.

"A National Acrobat" is propelled by one of Iommi's strongest guitar riffs, as well as by Butler's signature galloping bass lines. "Killing Yourself to Live" follows the model of "War Pigs" and "Iron Man" with a heavy, angry sound and a complicated song structure that progresses through several different movements. (Some fans say the lyric is inspired by Osbourne's pre-Sabbath days working in a slaughterhouse.)

Then of course there's the title track. "Sabbath Bloody Sabbath" boasts a killer opening riff--one of the heaviest that the band ever crafted--and it's made all the more effective by the mid-song acoustic detours. (Zep is always hailed as the champion of contrasting light and dark, heavy and acoustic, but here Sabbath proved that it was just as adept at that trick.) Meanwhile, the concluding section approaches progressive rock in its complexity (though Yes or Genesis could never inspire such head-banging).

According to Butler, the epic track came together in a sudden burst of inspiration. "Tony started playing 'Sabbath Bloody Sabbath,' the main riff, me and Bill just came in on bass and drums, we just played together and the whole thing materialized," he told author Mike Stark in the book Black Sabbath: An Oral History.

The lyric ranges from a positively vitriolic rage ("The people who have crippled you/You want to see them burn," Osbourne howls, his vocals still in prime form) to an uncharacteristically resigned introspection ("Nobody will ever let you know/When you ask the reasons why/They just tell you that you're on your own/Fill your head all full of lies"). They build to a climactic shout of "you bastards!" leading into a searing guitar solo and an instrumental rave-up that are connected by a classic falling-down-the-stairs Ward drum fill.

Butler, the group's primary lyricist, told Stark that the theme of the song was a bitter attack on managers who'd betrayed the band, as well as a musing on the illusory nature of the rock-star dream. "It was about the attitude of certain people," the bassist said. "The British press and people like that, you know, really slagging us and everything, and doing so many tours ... all the glamorous thing was gone for us. It'd been beaten out of us by then. 'Sabbath Bloody Sabbath' is just our attitude toward the music business--the business side of things."

Added Osbourne: "I'd rather not talk about 'Sabbath Bloody Sabbath.' It was a very personal thing, and I wouldn't like to embarrass the person it was written about."

Nearly 30 years on, listeners don't need to know exactly who Sabbath was railing about to appreciate the tune's power. Like all timeless rock anthems, it stands as an inspired protest against conformity in all of its many guises. And while the group would arguably never top this accomplishment again, it remains an energizing example of a groundbreaking and justly revered band at the very peak of its hard-rocking powers.