Natalie Portman sighting on set of Thor 2 on November 21, 2012 in London, England. Credit: Simon James/FilmMagic

Natalie Portman's not-so-incognito anymore.

The Oscar-winning actress, 31, resurfaced in London to begin shooting Thor: The Dark World -- and, appropriately enough, Portman has gone back to her dark roots!

To reprise her role as Jane Foster, the earth-dwelling romantic interest of hammer-swinging Norseman Thor (Chris Hemsworth), the star returned to the natural brunette hair that made her famous in movies like Star Wars Episode One: The Phantom Menace and Black Swan.

But the Israeli-born stunner underwent a startling hair makeover back in September, going bombshell blonde for her sexy role opposite Michael Fassbender in an untitled Terence Malick film, which wrapped up shooting in Austin, Tex. last month.

"Natalie makes a beautiful blonde!" her colorist, Tracey Cunningham told Us Weekly. "She needed to make the change for an upcoming movie role, and using Redken's Blonde Dimensions we achieved a gorgeous, multi-dimensional golden blonde."

It's been an eventful year for Portman, who wed star dancer-choreographer Benjamin Millepied, father to her son Aleph, 17 months, in Big Sur, Calif. in August.

"She has this glow about her these days," a pal recently observed. "There's been a change in her since the little man arrived. She and Benjamin are so content."