Before I've looked at indirect measures of what hair color guys prefer on a girl when their brain is in sex mode, such as what fraction of the Maxim Hot 100 or some similar list is blond. (Go to GNXP.com and search "brunette.") Still, it would be more informative to see directly how guys vote with their time and clicks, rather than who a group of magazine editors deems attractive, where we have to infer demand from supply.
There are at least two sources, and they gave pretty similar answers, so I won't look for other similar ones. One is the list of most popular girls at Freeones, which allows you to search for someone by name and retrieve a list of links to pictures or video clips featuring her. They don't say exactly how popularity is determined, just that it is "from the search engine and the amount of clicks a model gets." I assume that means they take the number of times guys search for her, as well as the number of times her links are clicked, and weight these somehow. There are 100 girls listed.
The other source is the list of most viewed girls at Keez Movies, which is like a porn version of YouTube. Their videos have a tag for which girls are shown in the clip, so they can tell how many times any video featuring a particular girl has been viewed. As far as I can tell, the ranking is based just on these view counts, not searches. There are tons of girls listed here, so I just went with the first 100 to make things similar to the other source. (The mix of hair colors might change if we went from the top 100 to the top 1000, for example.)
I classified girls as either light or dark, which was fairly easy because there were very few in-between cases. My guess is that a lot of the blonds have dyed hair because it looks that way and because the gap in color is so extreme. If they were mostly natural blonds, there would be a continuum from platinum to dark blond, but they're almost entirely bright blond. That makes sense since entry into the porn industry selects for wild personalities, and brunettes are wilder. Some of these dark-haired girls then decide to bleach their hair for niche appeal. There was only one redhead, and I classified her as light.
At Freeones 62% are dark, while at Keez Movies 66% are dark. I give stronger weight to the Keez Movies figure because each girl's rank is based on millions or sometimes over 10 million views, while each girl's rank at Freeones is only based on roughly 1000 votes. Plus video clips more directly tell us what guys want when their brains are in sex mode, since most guys want to see moving rather than still images. So let's say that 65% of the most popular porn girls have dark hair.
In Northern Europeans, blonds make up about 16% of females. There are no data for Americans. * So blonds are a little over twice as common as you'd expect if hair color did not matter at all, but they're still a minority. While "twice as common as chance" sounds big, I see that as a small effect relative to other effects such as skin tone, breast size, butt perfection, and so on, all of which are highly related to age. Girls who have a nice ass are more than just twice as common among porn girls compared to the overall female population, and ditto breast size / shape and skin tone -- probably more like 100 times more common. (Remember to factor lower-class and trailer park people into your composite picture of what the average female looks like.) There's no shortage of blonds, so if that trait by itself were as powerful as skin tone, virtually all popular porn girls would be blond.
So there is a positive effect of light hair on sex appeal, but its strength is weak compared to those of physical traits that we commonly think of as determining sex appeal.
* I don't think it's correct to say that the blond percentage for Americans will be so much lower than for Europeans because of the large Black and Hispanic populations we have. That move would assume that Black and indigenous-looking Hispanics were even in the pool to choose from in the first place. There are virtually no popular Black porn girls, and almost all of the Hispanic ones look European. I see that filter as applying first -- for whatever reason, the average porn viewer isn't interested in girls of those races (white-looking Hispanics are fine, though). It's only after this screening occurs that managers choose based on skin tone, breast size, hair color, etc.
If only these latter traits mattered, you'd see a fair number of popular Black porn girls. They might sport straightened hair, but they'd be there. So it seems like there's a preliminary filter that allows in Europeans and only a handful of blacks and indigenous-looking Hispanics. That's why I use the blond percentage among Europeans, not whatever the lower percentage is among all Americans.