I know historically Circassians and Georgians were regarded to have had very beautiful women (https://en.wikipedia.org/wiki/Circassian_beauties), even though today when I look at them they don't strike me as being above-average in attractiveness. Today I guess it's the Scandinavians people regard to be very beautiful, and I'm wondering whether this is a recent stereotype? How long has this stereotype of Scandinavians existed? Historically have there been any countries or regions in Europe or in the rest of the world that Europeans typically believed had the most beautiful women?
Would this also depend on when stereotypically Scandinavian traits (e.g. blond hair, blue eyes, height, slenderness) began to be considered attractive?
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Not sure if this answers directly to your question as you're asking about the existence of a stereotype, but from what we know about the Eurasian slave trade we could get at least some idea how blond Nordic people were seen in the southern parts of medieval and pre-modern Eurasia.
A relevant paper: The Baltic Finnic People in the Medieval and Pre-Modern Eastern European Slave Trade
Also this blogpost gives context and details with more sources: Blonde cargoes: Finnish children in the slave markets of medieval Crimea
Crimea was the center of the Black Sea slave trade. In the Crimean markets blonde girls and boys captured from Finland and Karelia were the most valued slaves because of the color of their skin and hair. And their exceedingly high value made it worthwhile to capture them in the North and to transport them to these far-away markets.
The prices they commanded, however, were simply colossal; one source notes that girls who could be purchased for as little as 5 altyn in Karelia could be resold for 6,666 altyn even before they reached the Khanate – a mark up in excess of 133,000 percent. The higher price, equivalent to 200 roubles or (in about 1600) 250 sheep, was also about five times the usual price for a Crimean slave. It is no surprise, in these circumstances, that slaves from the far north were highly sought-after for their colouring – nor that their special characteristics were scrupulously noted in the slave registers so carefully kept in the ports that lay at the heart of this commerce in human misery: “white skin, white hair.”
Of course, there are many factors into a price: rarity, exotism, luxury etc. yet the high prices tell us these slaves were also valued for their beauty. And the participants on the slave markets must have been informed about them as there was networks for capturing, transporting and trading these blonde slaves.
So the clientele knew about these people, sometimes even seeking after them in the markets (for example, Shāh Abbās of Persia sent out delegations to slave markets that acquired three Finnish girls in Moscow and 30 more in Kazan), the slave traders knew where to get them, and raids were extendend as far as the coast of the Gulf of Bothnia in Finland to capture blonde boys and girls.
But I'm not sure if that qualifies as a stereotype in the sense you were asking about.
I'll answer part of your question. In Hollywood, the "blonde bombshell" female ideal which is ascribed to Scandinavian women took hold ca 1930-1933. The 1920s were the era of the boyish and flat-chested Flappers, who had short, dark hair - in 1933 silver-blonde Jean Harlow broke through to be the #1 female film star, and Fay Wray got to wear a blonde wig for her role in King Kong.
Swedish-born Greta Garbo (who had light brown hair) was already a star, topping the charts with Grand Hotel in 1932, and Carol Lombard had her hair bleached for a number of hit comedies from 1930-1936.
Sources:
This is a stereotype popular mostly in the USA, and it is more nuanced than that. It encompasses the idea that Scandinavians (particularily Swedes) are beautiful, kind of naive, innocently sexually attractive and sexually liberated.
Its part of a larger stereotype that Europeans are more sexually liberated (and presumably desirable), than Americans. (Notably, this stereotype is not particularily popular in Europe, where the role of a "sexy foreigner" in popular media is usually filled by Slavic women and Mediterrean men, not Scandinavians.)
Its hard to pin an exact date on when this stereotype appeared, but its most likely after 1960, when examples of (often sexually explicit) Scandinavian cinema reached the US.
Source:
Scandinavian Blue: The Erotic Cinema of Sweden and Denmark in the 1960
Rohner, Jean-Christohpe; Rasmussen, Anders. "Recognition bias and the physical attractiveness stereotype". Scandinavian Journal of Psychology 53