The phrase blonde moment denotes an instance of being silly or scatterbrained.

It refers to the stereotypical perception of blonde-haired women as unintelligent.

The American screenwriter, playwright and author Anita Loos (1889-1981) mentioned this stereotype in an interview about her new novel, Gentlemen Prefer Blondes, in which she chronicled the Jazz Age—this interview, by Marguerite Mooers Marshall, was published in the Springfield Missouri Republican (Springfield, Missouri) of Sunday 8^th November 1925:

“The blonde—the typical one—considers brains excess baggage. That’s why she’s so easy on the tired business man. He doesn’t have to be intelligent to get along with her. Men, the majority of them, do not like clever women for their hours of ease. They want sweet, superficial, silly creatures—and the blonde usually fills the bill.
“Of course,” Miss Loos added in an apologetic parenthesis, “there are, now and then, blondes that are not feather-headed, like the canaries they resemble; blondes who have brunette brains. Some of them even have brunette temperaments. But they are not nearly as successful with men as their ‘beautiful and dumb’ blonde sisters.”

The earliest instance of blonde moment that I have found is from Announcements and Notices, in the Rapid City Journal (Rapid City, South Dakota) of Saturday 17^th August 1991:

WE had a blonde moment while Ms. Violet was on vacation & erased all the phone messages. So sorry. Try again.
Moonbeam Agency, 342-1409

The second-earliest occurrence that I have found is from a letter published in The Santa Fe New Mexican (Santa Fe, New Mexico) of Wednesday 18^th December 1991:

‘Mental hold’
As I read my Nov. 29 paper, I was shocked by Ellen Corbet’s attack on K. C. Compton. How anyone could find an article as fun-loving and light-hearted as Compton’s offensive I do not understand.
I respect Ms. Corbet immensely for trying to protect some people who may be unable to protect themselves. However, this article was not meant to harm anyone or make light of those of us who are not always on an even keel.
I understand about Ms. Corbet’s “mental hold;” I’ve been there under slightly different circumstances. It is very frightening to have someone threaten you with the possibility of “sending you state” (State Hospital). It happened to me.
Ms. Corbet, save your strength for someone who is trying to hurt us, not someone who was set off on an amusing train of thought by something she read.
My blessings to anyone who wants to “go on mental hold” or have a “Blonde Moment.” I do it all the time. It’s simply another way to deal with a reality that sometimes gets a little hard to take.
                                                                                                                                     Dawn Peter
                                                                                                                                          Santa Fe

In Dawn Peter’s letter therefore, blonde moment connotes a positive mental attitude. Likewise, in the following personal advertisement from Seeking Males, in The Tampa Tribune (Tampa, Florida) of Friday 8^th July 1994, does have blonde moments reinforces the woman’s attractiveness although (or rather, because) the phrase is contrasted with intelligent blonde:

CLASSY, UNIQUE, intelligent blonde, but does have blonde moments sometimes, 48 and fit. ISO 45-55, fit, 6’, n/s, professional Male into sports, art, music; who’s witty, not afraid to show his feelings, who can laugh at times and yes, even cry, and who likes to have fun.