Bob Rock produced The Black Album and later played bass on Metallica's St. Anger.
Rock's first Metallica production was the 1991 self-titled album. He was already known for producing major hard-rock records, and his work helped give Metallica a heavier but more controlled and radio-ready sound.
When Jason Newsted left Metallica in 2001, Rock performed bass on the band's 2003 album St. Anger. He was a temporary studio participant rather than the band's permanent bassist; Robert Trujillo joined Metallica later in 2003.
The two roles are easy to separate chronologically. Rock was behind the console for the Black Album, while his bass performance belongs to the later St. Anger sessions.