"It's something she's wanted to do for a long time," her colorist George Papanikolas said. "I've been coloring her hair for a year, and she's wanted to go blonder since the day we met," he told E! Online.
And he has confirmed that Kim's partner Kanye West is a big fan. "He loves it! He was a big cheerleader for her to go blond".
Kim, who gave birth to her baby daughter North in June, had a clear idea in mind for her radical hair transformation, showing examples of hair colourings to George.
"She was so excited. She loved it!" Kim's hair colorist said. "It was scary for me!"
It's not the first time the new mum has turned blonde. Kim also left her brunette locks behind in favour of lighter tones back in 2009, and again in 2012 when she tweeted, "Blonde hair don't care".
This time, George explains that the 32-year-old has gone for a "golden honey blond."
Two appointments were needed to complete the new look, which took place at Kim's mum Kris Jenner's house, making it a real Kardashian affair.
"The whole family was watching. Then, everyone wanted to go blonder!" says George, from the Andy LeCompte salon in Los Angeles.
"We kept it really modern and rooty," he says of the style, which also included a cut from Chris McMillan, who is Jennifer Aniston's hairdresser.
What do you think of Kim's new look? Blonde or brunette?