I thought this was going to be about Prince’s The Black Album.
It’s very possible that Metallixa was influenced by Spinal Tap for theri album cover, but other all-black covers had been done before. Comedian Murray Roman’s album Blind Man’s Movie, from circa 1969 or '70, not only had an all-black gatefold cover, but even had all-black pages bound inside. There was also a popular Beatles bootleg that closely parodied the “White Album” cover (complete with inserts and embossed logo), only in black.