Bob Rock was the newly hired producer for Metallica's 1991 album, commonly called The Black Album.
Metallica had been impressed by Rock's work on Mötley Crüe's 1989 album Dr. Feelgood. The band brought him in to help shape a record that sounded more powerful, focused, and accessible than its earlier releases. Rock also received producer credit alongside James Hetfield, Lars Ulrich, and Metallica.
The collaboration was not easy. Rock pushed the musicians to record more collaboratively, capture live energy, improve the lyrics, and make the bass more audible. The recording process involved repeated arguments and many takes, but the finished sound became one of the album's defining features.
Rock continued working with Metallica on later projects, including Load, Reload, Garage Inc., and St. Anger. The Black Album therefore marks the beginning of a long production relationship, not merely a one-off studio assignment.