So let me try to explain this.
A man is a blond.
A woman is a blonde.
Did you notice the A in there?
When used to describe a person, you follow the above rule.
The blond ordered a cup of coffee. *That sentence just informed me a man with blond hair bought a cup of coffee.*
The blonde ordered a cup of coffee. *That one just told me a woman with blond hair bought a cup of coffee.*
BUT, when used as an adjective, blond hair has no sex. Look how I used it above when explaining sentence number 2. (woman with blond hair...)
She brushed her light blond hair.
He ran a hand through his blond locks.
The wig was blond.
It's really very simple. Unless you're having a blond moment... ;)
(Note. That's a joke. I do not in any way or form believe the blond people theory.)