The arrival of a rival dancer, played by Mila Kunis, triggers both obsessive jealousy and sexual liberation in a plot echoing that of the ballet around which it revolves.
A steamy love scene between the actresses and elements of violence and horror make it a departure from clean-cut on-screen roles often associated with Portman.
"(Director) Darren (Aronofsky) talked to me about this (sex) scene in our first meeting eight years ago," Portman told reporters in Venice after a press screening.
"He described it as: 'You're going to have a sex scene with yourself,' and I thought that was very interesting because this movie is in so many ways an exploration of an artist's ego and that narcissistic sort of attraction to yourself and also repulsion with yourself."
Portman, her co-stars and the Venice jury led by director Quentin Tarantino walked the red carpet at the official world premiere, signing autographs and mingling with hundreds of fans.
"I just want to say 'amore cinema'," Tarantino told the opening ceremony.
Aronofsky won the top prize in Venice -- the Golden Lion for best picture -- two years ago with "The Wrestler," and he said he saw similarities between it and Black Swan.
"The more I looked into the world of ballet, I actually started to see all these similarities to the world of wrestling -- they both have these performers that use their bodies in extremely intense physical ways."
French actor Vincent Cassel, who plays the ballet director, wondered why anyone would want to go into the world of dance.
"I think if you want to be a dancer it has to be a vocation. It's like being a priest, really, because you work so hard, you work every day, it hurts like crazy and you make no money. So I guess it's just not something one should do."
YOUTH OVER CELEBRITY
Black Swan kicks off the annual Venice film festival on the Lido waterfront where stars, fans and reporters rub shoulders for the next 11 days.
Festival director Marco Mueller has opted for youth in his choice of directors of the 23 competition films, and he will also hope that the presence of Hollywood mavericks can make up for the expected shortage of A-list celebrities this year.
The average age of filmmakers in the main line-up this year is an unusually low 47, and includes Oscar winner Sofia Coppola, 39, with comic drama "Somewhere."
At the other end of the age range are 78-year-old Monte Hellman, competing with low-budget crime drama "Road to Nowhere," and Polish filmmaker Jerzy Skolimowski, 72, on the Lido with the Afghan-themed "Essential Killing."
Actor Casey Affleck presents documentary "I'm Still Here," about his brother-in-law actor Joaquin Phoenix's decision to retire in 2008 and reinvent himself as a hip-hop musician.
And Julian Schnabel directs "Miral," about an orphaned Palestinian girl growing up in the wake of the first Arab-Israeli war, who finds herself drawn into the conflict.
One filmmaker who will not be in Venice despite having a film at the festival is Jafar Panahi, the Iranian jailed and later released earlier this year. He said in a statement he had not been given permission to travel abroad by Tehran.