Black Sabbath's perception has changed greatly over the decades. Back in the 70ies, they weren't really seen as a band with a musical contribution, more as a one hit wonder (Paranoid) with a mad singer you couldn't take entirely serious. That only changed a little with
Sabbath Bloody Sabbath (the album), their most orchestral and arranged work (Vol. IV already indicated that direction). Soon after though, Punk happened and Sabbath and their sluggish riffs (no doubt an art, but not valued as one back then) were dubbed dinosaurs. When Ozzy left/was fired (after the Dave Walker intermezzo), they were viewed as a spent force that had seen its ass kicked by an upstart opening act named Van Halen (with a very nimble guitarist who had all digits intact) on the European tour.
Even during the Dio years - musically nothing to be ashamed of -, they were still smiled upon, Ronnie's lyrical sword & sorcery obsession never endeared him to critics. Gillan joining (and I think
Born Again is one of the best albums Gillan has sung on outside his Deep Purple canon, I also think that this album and its production - derided at the time, a sonic classic by today's standards - was an unsung trigger for Grunge music as a whole) was viewed as a hilarious move spawned by and in drunken stupor (well, it actually was!
).
After that, with constantly changing line-ups, it was diminishing returns, though not without highlights.
It was only the advent of Grunge - not the LA glam hard rock before which patterned itself more after Led Zep and Aerosmith - that led to a lasting re-appreciation of Sabbath and a reinstatement of theirs into the "holy trinity" of Brit heavy rock that is Led Zep, Purple and the Sabs. I remember my surprise when all these Grungies all of the sudden voiced admiration for Sabbath's
music (and not just Ozzy's showmanship) come the 90ies. Before that, mention of Black Sabbath was invariably always tied to some joke on them, even in metal mags. Now, the sheer physical necessity of a handicapped guitarist (and closet Joe Pass fan) to downtune co-led to a new musical movement. Life writes the most amazing stories.