Which outside producer did Metallica hire for The Black Album?

Metallica hired Bob Rock as the outside producer for The Black Album.

The band had been impressed by Rock's work on Mötley Crüe's 1989 album Dr. Feelgood. Although Metallica initially hesitated about having him produce the record, the members later decided that he could help them make their strongest album.

Rock encouraged the group to record collaboratively rather than building parts separately. He also pushed for live-feeling performances, harmonic vocals, and a more audible bass sound than on ...And Justice for All.

The sessions were difficult. Rock and the band frequently argued about lyrics, performances, and production choices, and he later described the experience as highly stressful. Despite the tension, the partnership produced Metallica's biggest commercial success and continued on later releases.