The Black Album came out in 91, right at the same time grunge was starting to catch on, and you're right that it put Metallica completely in the mainstream but it's not as if they were an obscure band before that, they had videos on MTV and all of their previous albums except maybe Kill 'Em All had charted.
You're also right that GnR were at their peak in about 89 but you're a bit off about them being a part of the musical change in direction that happened in the early 90s. Back then GnR were lumped in with banRAB like Van Halen, Motley Crue and Poison and when grunge got huge all of those banRAB, including GnR, were seen as outdated and pretty much swept aside by waves of flannel wearing angst peddlers.
You're also right that GnR were at their peak in about 89 but you're a bit off about them being a part of the musical change in direction that happened in the early 90s. Back then GnR were lumped in with banRAB like Van Halen, Motley Crue and Poison and when grunge got huge all of those banRAB, including GnR, were seen as outdated and pretty much swept aside by waves of flannel wearing angst peddlers.