The Black Album is a common reference to Metallica's eponymous fifth album.
Before Metallica released this album, which has a black matte cover with a black satin impression of a coiled rattlesnake (reminiscent of that seen on the Revolutionary War flag designed by Christopher Gadsden with a coiled rattlesnake on a yellow field with the legend "Don't Tread on Me" which is also the title of one of the songs on the Metallica album) The Black Album referred to the then only release by the band Spinal Tap.
Spinal Tap's first album is titled This is Spinal Tap after the "rockumentary" of the same name. It is supposedly a collection of their greatest hits. The album itself has a featureless black cover.
The reason the Spinal Tap album has a black cover can be found in the rockumentary. In the rockumentary, Spinal Tap released an album titled Smell the Glove which featured cover art of a woman on her hands and knees wearing a collar and leash being made to smell a black leather glove as if she were some form of hunting dog. Unfortunately for the band, their record label found this image to be too offensive and it was replaced with a plain black cover. The band opposed this vehemently, but they lost that battle and so the black cover it was.
The original rockumentary was released in 1984, directed by Rob Reiner and starring Christopher Guest, Michael McKean, and Harry Shearer as the legendary heavy metal band. The album was also released in 1984. Eventually, with the continued popularity of both, there was a second album released in 1992, Break Like the Wind.