Where were the main recording sessions for Metallica's The Black Album held?

The main recording sessions for Metallica's The Black Album were held at One on One Recording Studios in Los Angeles.

Metallica began recording there in October 1990, working with producer Bob Rock during sessions that lasted roughly eight months. The studio became the main setting for the band's effort to create a more cohesive, live-feeling album than its earlier records.

The project also involved a short period at Little Mountain Sound Studios in Vancouver, British Columbia. Near the end, additional overdubs and mixing took place at A&M Studios in Hollywood. Those later locations are why descriptions of the album's recording history can mention more than one studio.

One on One is therefore the best answer when a question asks for the principal recording site. It is different from the Hollywood studio associated with final overdubs and mixing, and from the mastering facilities used for later editions.