Few knew when David Bowie released "Blackstar" on Friday that it would be his final album -- but it is a mysterious and sombre testament which critics hailed as his best work in years.

Inventive to the end, Bowie, who died on Sunday aged 69, mixed rock and jazz on an album for which the famously private star gave no interviews and whose cover was a simple black star on a white background.

With song titles including "Lazarus" and "I Can't Give Everything Away", Bowie seemed to be pondering major themes on his 25th studio album, though critics' efforts to read between the lines did not get very far.

"It's a rich, deep and strange album that feels like Bowie moving restlessly forward, his eyes fixed ahead: the position in which he's always made his greatest music," the Guardian's review said.

The New York Times, meanwhile, described the album, released on Bowie's birthday, as "at once emotive and cryptic, structured and spontaneous and above all, willful".

Tony Visconti, Bowie's long-term producer, called "Blackstar" -- which ran to just over 40 minutes long and featured seven songs -- the star's "parting gift".

In retrospect, some of the lyrics could be seen to refer to mortality, though not in any straightforward way.

The 10-minute title track, "Blackstar", refers to "the day of execution", prompting fans to speculate that it could be about anything from the rise of the Islamic State (IS) group to 20th-century occultist Aleister Crowley.

"Lazarus" -- named after the biblical character who rose from the dead thanks to a miracle carried out by Jesus -- opens with the lines: "Look up here, I'm in heaven/I've got scars that can't be seen/I've got drama, can't be stolen/Everybody knows me now."

Its final verse concludes: "Oh I'll be free/Just like that bluebird/Oh I'll be free/Ain't that just like me."

However, seasoned Bowie-watchers have always cautioned against trying to read too much into the star's lyrics, particularly on more experimental albums such as "Blackstar".

The New Yorker's review noted it was "rare for Bowie to embrace clear meaning", highlighting one of his most famous albums, 1974's "Diamond Dogs", as an example.

The singer himself seems to acknowledge this point in the album's final song, "I Can't Give Everything Away".

"Seeing more and feeling less/Saying no but meaning yes/This is all I ever meant/That's the message that I sent," the lyrics say.