Which producer was newly hired for Metallica's The Black Album?

Bob Rock was the newly hired producer for Metallica's The Black Album.

Metallica had been impressed by Rock's production work on Mötley Crüe's 1989 album Dr. Feelgood and chose him to help shape their next record. Rock's involvement affected the band's recording process: he encouraged the members to perform more collaboratively, record tracks live where appropriate, and pursue a fuller, more forceful sound.

The partnership was not effortless. Rock pushed the band to make numerous takes and to improve aspects of the songwriting and lyrics. The demanding sessions produced frequent arguments, and the experience was stressful for everyone involved. Nevertheless, the finished album became the band's commercial breakthrough.

Rock was not the only person credited in production. James Hetfield, Lars Ulrich, and Metallica also received production credits. The question refers to Rock because he was the outside producer newly brought into the project and the central figure associated with its recording approach.