Actually the black album is sort of interesting in this regard because while it was a really strong album it represented a sound that ultimatly failed the band. So in a sense the change in sound was "jumping the shark" even though the album itself was really solid. Or at least that's my take.

Certainly an interesting take, although I disagree. I don't think that it was the new sound itself that caused their downfall. That sound showcased Thrash in an new sociably acceptable manner without changing the music much itself. Nowadays it is so easy to consider that combination of sound and music run-of-the-mill, but at the time of the release it was still right on the fringe. It still took a couple of years for it to become totally accepted in general society. It's probably fair to say that the album was responsible for changing many peoples' conceptions of what 'that heavy metal shit' really was.

Unfortunately with that acceptance, Metallica became part of the very system that they had spent their entire career denigrating, and also took them from being very successful niche muscians to megabuck-earning stadium-fillers. This transition appears to be complete, witness their stance on p2p filesharing. Can you imagine Metallica of 1986 being so anti-p2p? I think that everything that Metallica has written since the black album has been written from that new perspective. They sold out.

I don't think that it's fair to suggest that the album's sound failed the band in any way, or to denigrate the album because of that perception. If anything, the band failed the sound. They could have kept riding the fringe of social acceptability and pushed the frontier further whilst retaining musical integrity. They could have even backed off into older realms and pushed some of their new fans away to ensure that was the case. They chose instead to take the easier path and sell more albums.