There is variation in usage. The word "blond/blonde" has been used in English since at least the middle ages, but was dropped from common use until it was re-introduced from French, relatively recently.
When words are borrowed from other languages, sometimes elements of that language's grammar are borrowed too. (for example, we use the Latin plural of words like "cacti", but the regular plural, "cactuses", is also possible)
The rule in French is for the adjective to agree in gender with the noun. Now English nouns don't have gender, but when describing a woman, the word is often spelt using the French feminine gender. And this seems to be the most common spelling. It seems that this spelling is becoming dominant. As evidence of this, my spell checker is asking me if "blond" is a typo for "blood"; it thinks "dblond" is a rare word, and may be an error.
Some would think it is a mistake to write "a blonde man", I prefer to think of it as an example of language change.
For your own writing, I'd still recommend using "blond" for men and "blonde" for women as nobody is likely to criticise this as being wrong (except my spell checker, which still wants me to write "blood")