The official title of Metallica's 1991 album nicknamed The Black Album is Metallica.
The record is self-titled, meaning it uses the band's own name rather than a separate phrase. Fans adopted The Black Album because the original packaging was dominated by black artwork, with a dark Metallica logo and a coiled snake that were difficult to see against the background.
The nickname also helps distinguish this album from the band's wider discography. Metallica had already released Kill 'Em All, Ride the Lightning, Master of Puppets, and ...And Justice for All before the self-titled record appeared.
“Metallica Black Album” is therefore a common informal name, not the formal title printed as the album's title. It is also unrelated to Prince's album The Black Album, which is a different release by a different artist.