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As a noun, platinum blonde designates:
– a person (especially a woman) with a silvery-blonde hair colour;
– a silvery-blonde hair colour.

As an adjective, platinum blonde means:
– of a person: having silvery-blonde hair;
– of hair: silvery-blonde in colour.

The expression platinum blonde occurs, for example, in Size zeros can’t measure up to double ‘D’ Diana Dors, by Patrick Newley, published in The Stage (London, England) of Thursday 10^th May 2007 [page 52, column 2]:

Not surprisingly, one of my dad’s favourite actresses—and mine—was the late and much lamented Diana Dors, Britain’s answer to Marilyn Monroe and Jayne Mansfield, and a lady who was variously described as being ‘amply proportioned’ or ‘the hurricane in mink’.
DD—as she was nicknamed—was the pin-up for millions of men for three decades. Who could resist those platinum blonde looks, shapely curves and hourglass figure?

These are, in chronological order, the earliest occurrences of platinum blonde that I have found:

1-: From the Daily News (New York City, New York, USA) of Tuesday 25^th January 1927 [page 14, column 1]:

Here Are Methods For Keeping Blonde Tresses Golden
by Antoinette Donnelly

There is a blonde type to me more irresistibly lovely than all the rest of the women who come under the “preferred” classification. She is the platinum blonde. And when she has been endowed, additionally, with eyes of violet hue and the pastel coloring extends to skin and colors she choses [sic] for her dress, she is the real dream girl, if there be such.
She is rare. There are thousands of blondes with gold in their hair, and as many with red, and quite as many again with the yellow of corn, sometimes real and sometimes corned with the aid of a well known bleaching ingredient—but the platinum blonde maintains her supremacy by her rarity.

2-: From The Winnipeg Evening Tribune (Winnipeg, Manitoba, Canada) of Wednesday 30^th March 1927 [page 14, column 4]—this text also contains the noun platinum in the sense of a silvery-blonde hair colour:

ORPHEUM—Who should know better how to take care of blonde hair than Ida May Chadwick, herself one of the most picturesque blondes in the theatre? Miss Chadwick is at the Orpheum theatre this week with her company of Six Dizzy Blondes. […] “The worst worry of a blonde is how to stay blonde,” says Miss Chadwick. “Unless they are peroxide and can brighten up regardless of nature, the fair-haired girl will experience the sorrow of having her locks darken as the birthdays mount up. In the early twenties a dusky shading sets in. Then some women resort to strong sodas, ammonia and borax to keep the color alive and this destroys the beauty of their hair by extracting the natural oils. The most beautiful blonde type is the ‘platinum.’ There are thousands with gold in their hair, many with red and yellow, but the real platinum blonde is a rarity.”

3-: From In Defense Of Blondes, by Marguerite Mooers Marshall, published in The Bulletin (San Francisco, California, USA) of Friday 26^th August 1927 [page C6, column 2]:

If we may be serious about blondes for a moment, every woman—even if she’s a brunette—has known and admired such blue-eyed sisters as our charming neighbor, as another, equally charming, who shines as housewife, politician and platinum blonde, as more than one indefatigable and efficient newspaper woman of our acquaintance, as the flapper we know best, who is attractive, intelligent—and blonde.

4-: From Gotham Daze, by William Lipman, published in The Minneapolis Tribune (Minneapolis, Minnesota, USA) of Tuesday 26^th August 1930 [page 6, column 6]:

Jean Harlow ¹ Gets Thrills in Big City
Quite the biggest of the current riots is Jean Harlow, the platinum blonde star of “Hell’s Angels,” ² who’s visiting Gothamtown for the first time in her young life. Miss Harlow made her film debut in the sky thriller on which Howard Hughes, young Texas millionaire, is said to have shot $4,000,000.

¹ Jean Harlow (born Harlean Harlow Carpenter – 1911-1937) was a U.S. film actress. She starred in Platinum Blonde (1931), a U.S. comedy film directed by Frank Capra (1897-1991).
² Hell’s Angels (1930) is a U.S. war film directed and produced by the U.S. industrialist, aviator and film producer Howard Hughes (1905-1976).

5-: From a review of Howard Hughes’s Hell’s Angels, by Robert F. Sisk, published in The Sun (Baltimore, Maryland, USA) of Sunday 31^st August 1930 [Section 1, page 1, column 1]:

The story isn’t worth a good hurrah. It detracts from the film because it is, for the most part, cheap and uninteresting stuff in which the heroine is a bleached blonde and as artificial looking as a stage moon. Her name is Jean Harlow, and she is advertised as a platinum blonde, which is simply another way of saying it. But since she is supposed to be an English aristocrat and because she attends a before-the-war ball in a dress which exposes too much, she never becomes very convincing.

6-: From Film Exploitation Dazes Broadway Press Agent, dated New York City, Saturday 30^th August 1930, by Charles Washburn, published in the Oakland Tribune (Oakland, California, USA) of Sunday 31^st August 1930 [page 6S, column 4]:

I strolled over to the picture offices. Hardly before I had my coat off a film reporter was on the telephone.
[…]
[…] I did tell him that “Hell’s Angels” had Croesus surroundings. “Hadn’t the feminine star, Jean Harlow, platinum blonde hair?” I said. “And didn’t I get the exploiting job because of my gold teeth?”