If you are describing a blonde woman and a blond man (or blonde women and blond men, or one of one gender and more than one of the other), as a pair/group are they blonds or blondes?
Realistically? No two ‘experts’ are going to agree and absolutely nobody else will care. Go with what looks best or flip a coin.
Taking it as a French adjective, the masculine form is used for mixed groups, so they would be blonds.
It depends on the writer. It really can be argued either way, unless you buy into the idea that language use is prescribable. But speakers and writers of a language will use whatever they want to use, and nothing is truly static when it comes to this sort of question.
Bilingual people (or monolingual English-speakers who have researched gendered nouns in other languages) are likely to be more picky about the blond/blonde distinction once they’ve learned it exists. But I suspect the majority of Americans (who are monolingual English-speakers) would say “the blondes” regardless of how many guys or girls are in the group.
The fact is, gender doesn’t stick well in English. Gendered words usually end up losing one spelling or the other as time goes by–note the relative rarity of the word “comedienne” these days. Yes, you know it means a female comedian when you see it, but the word “comedian” doesn’t just mean a male comedian–a comedian can be either male or female these days. And according to Wikipedia, “Increasing numbers of women are calling themselves actors rather than actresses, especially in the live theatre. The Screen Actors Guild annually gives out awards for ‘Best Male Actor’ and ‘Best Female Actor’.”
“Blonde” seems, to me, to be evolving into the de-facto spelling of the hair color in English. Even if that’s only because women are more likely to describe their hair color than men do. I have hardly ever seen the term “blond(s)” used in my daily life, but I see “blonde(s)” all the damn time–in salons, on boxes of hair coloring product, when guys express their preference for women with(out) yellow hair, jokes about dumb blondes, etc. It’s the reverse of the de-feminization listed above, but the point is English-speakers are in the process of going *from *two gendered words to describe men and women with yellow hair *to *a single neuter word to describe all people with yellow hair. I’m pretty sure “blonde” is winning, but only time will tell for sure. 
Here is a link where the issue is discussed elsewhere, you can google for more.
That said, if this is for a scholarly work or publication (newsletter, newspaper) where exact spelling matters, I’d go with “blonds” if it is a mixed-gender group containing at least one man. “Blondes” for a group of all women. For now, anyway.
Dictionary.com says “blond” is becoming more the default spelling for both sexes. Based on that, and the French standard, I’d go with “blonds”.
I prefer blondes.
That’s very gentlemanly of you!
It should be noted that, sexist as it is, it is very rare to use the noun form of that word to refer to a male. You might say “the blond guy”, but “the blond” sounds like you’re talking about a woman. “Those two blonds over there” would almost never be used to talk about a man and a woman.
I would avoid the issue and say “two persons with blond hair”.
Call them Blondies.
This may clear it up. But probably not. Spell it any way you like.