Lorde wore a gown from French fashion house Balenciaga for the Grammys ceremony.
The floor-length black dress - thought to be made from Japanese silk - is from the designer's pre-autumn collection.
Lorde wore her usual deep purple lipstick after a makeover by MAC Cosmetics senior artist Amber Dreadon, who told the Los Angeles Times she wanted to "keep the look fresh and radiant - modern, a bit boyish and darkly cool with a focus on the lips".
Lorde's trademark brunette curls were straightened.
True to her indie style, she did not show up on the red carpet, where celebrities are typically asked which designer they are wearing, leaving many speculating on the Twittersphere.
For her performance of Royals, the Kiwi star wore a white sleeveless shirt by Italian fashion house Prada and loose black pants from French luxury goods brand Celine.
She had social media talking with her unusual choice of nail art.
Her simple monochrome outfit was teamed with black-smudged fingertips which looked as though they had been dipped in ink.
The odd fashion choice quickly prompted its own Twitter account as well as being likened to a symptom of the Black Plague by one US entertainment news website.
Lorde's fingernails - using the Twitter handle @lordefingernail - tweeted updates, with one saying "I would like to thank my thumb and pinky, without them, these black fingertips would not be possible".
That was followed by: "@lordemusic can split an atom ... with her fingernails. #justthetip."
And: "The #grammy party starts when Lorde's fingernails walk in. #partylikearoyal."
The look did not go down well with entertainment reporters, MTV describing them as "Black Death".
"During her performance of her hit song Royals, the 17-year-old singer sported black-tipped fingers, which are one of the symptoms of the European pandemic known as the Black Death, or the Black Plague," it reported.
Elsewhere, the Huffington Post asked its readers whether the fashion statement was "freakin' fantastic" or "totally terrifying".
It was slightly more positive though, describing the pop star's look as "bad ass" and "creative and a bit creepy".
Lorde had ditched the look when she took to the stage again to collect her award for best pop solo performance.
- additional reporting Patrice Dougan of APNZ
High school to hang out huge banner
Lorde's high school will celebrate her achievements by hanging a huge congratulatory banner on the main building for students as they return from their holidays today.
Takapuna Grammar principal Simon Lamb said: "Naturally, Takapuna Grammar school joins the local community in its support and admiration of our local talent, Ella Yelich-O'Connor. We are very proud of her achievements as recognised by the double ... win.
"We remain in close contact with Ella and her family.
"I am very proud of her, her music and her achievements."
Meanwhile, Lorde's former music teacher at Belmont Intermediate School, Jenny Bezuidenhout, watched the awards with her fingers crossed yesterday.
She cast the teenager as the lead in a musical she wrote called The Case of Greed, and also started the school's "Idol" competition which Lorde won when she was 12.
"So I'm sitting here saying to my husband 'Well, she sang my music before she sang her music'," Mrs Bezuidenhout said.
The teacher said she ran into Lorde's mother at a shopping centre before Christmas.
"She said, 'What do you buy a girl that's got everything?' I just said to her then, 'Ella is a gifted individual and she has the potential to really influence her generation through her art'.
"She inspires me ... she's really in touch with herself and how she feels about things and doesn't rely on the outside world to dictate how she should feel or think. And I really admire that in her because she's really young."
Her family had really nurtured her talent and had to take a lot of the credit, the teacher said.
"When she came to school last year, she's just the same as she ever was. She's just a lovely individual with an amazing presence and is very, very bright. She's got it all."