The Downton Abbey actress, famously known for her role as Lady Mary Crawley in the hit ITV period drama, had her dark brunette locks styled into a sleek bob to contrast against the striking number.
Her darker-than-usual tresses contrasted against her pretty pale complexion and matched her black clutch bag and sky high heels.
She also showed off her natural beauty with minimal make-up as she walked out on to the red carpet at the Miu Miu party.
A far cry from the period costumes she dons to play Lady Mary, the 31-year-old was striking as she was dressed head to toe in Italian fashion house Miu Miu.
But she will soon be back in dresses dating back to Britain in the 1920s when the doors of the award-winning Downton open for a fourth series in September.
Set six months after Lady Mary's loss of Matthew Crawley, a change of pace will see potential suitors come flooding in for the new widow.
The London-born star, whose character gave birth to baby George only to tragically lose her husband Matthew in a road accident, recently spoke of the various love interests to come in the surprising turn of events.
She told a Television Critics Association panel at the Beverly Hilton Hotel: "The fourth [season] is about recovery. She's starting a new life."
New love interests include Charles Blake, played by Foyle's War star Julian Ovenden, 36, and Lord Anthony Gillingham – "an old family friend… since the girls were little", played by Welsh actor Tom Cullen.
Another handsome gentleman to cross the threshold is Evelyn Napier, played by Brendan Patricks. He was seen in season one when he introduced Mary to her first lover, Turkish diplomat Mr Pamuk, who died alongside Mary in her bed.