When the gatefold is opened with the vinyl removed, and a strong light shone onto it from behind, the front of the album is illuminated by a field of stars.
One user of the image-sharing site Imgur discovered this by accident when he left the album exposed to the sunlight.
Jonathan Barnbrook, the album's designer, has previously talked about the meanings behind the "Blackstar" design, especially mortality.
"The idea of mortality is in there, and of course the idea of a black hole sucking in everything, the Big Bang, the start of the universe, if there is an end of the universe," he told Dezeen
. "These are things that relate to mortality.
"It's subsided a bit now, but a lot of people said it was a bulls**t cover when it came out, that it took five minutes to design. That's also what they say about 'Next Day' [another Bowie-Barnbrook collaboration]. But I think there is a misunderstanding about the simplicity, about how it actually takes a long time to get there and that simplicity can be a little more eternal in its meaning."
Barnbrook had a longstanding collaboration with Bowie, working with him on the design of four other albums: "Heathen" (2002), "Reality" (2003), "The Next Day" (2013) and the career retrospective "Nothing Has Changed" (2014). He could not be reached for comment.
The discovery adds to the mystery surrounding "Blackstar," which fans and critics alike see as a last parting gift from the artist, with hints of his impending death from liver cancer peppered throughout the lyrics -- "Something happened on the day he died/ Spirit rose a meter and stepped aside/ Somebody else took his place and bravely cried/ I'm a Blackstar -- and the music videos accompanying them.
"Blackstar" was released on January 8, two days before Bowie's death, and the initial run of 5,000 vinyl albums sold out in the UK four days after the release.