Metallica was the band's first album to debut at number one on the Billboard 200.
Released on August 12, 1991, the self-titled record arrived after three earlier albums had built the band's reputation without reaching the top of the U.S. album chart. Its debut marked a major commercial breakthrough and helped move Metallica from the metal underground into the global mainstream.
The album sold 598,000 copies in its first U.S. week according to the cited album history, and it also debuted at number one in several other countries. Its chart success continued beyond the initial release, with the album spending four consecutive weeks at the top of the Billboard 200.
The answer is Metallica, not The Black Album. The Black Album is the common nickname, while Metallica is the official title used in charts and discographies. The record followed ...And Justice for All and preceded Load, which later also reached number one.