Why shouldn’t you describe male characters as "blonde"?
The short answer is "Latin."
English is an old language with a long and storied history of beating up other languages and stealing their lunch money borrowing words from other languages, especially Latin and French, as both Rome and France occupied England for significant chunks of history.
Latin gave birth to French, which, like many other languages throughout the world, likes to assign its nouns gender. Everything has a gender: the car (feminine), the table (also feminine), your eyes (masculine).
The adjectives that describe those words also have gender, so a man with yellow hair is blond, and a woman with the same hair is blonde (an added 'e' at the end of the word usually indicates the feminine in French, like in fiancé and fiancée). Both words are pronounced the same way, which can make it harder to remember the difference.
"Yeah, she was gorgeous," Gojyo said. "Tall and blonde, with legs up to here--"
"To where?" Goku asked.
"I've always wondered that," Hakkai mused, thoughtfully.
"Anyway, turns out she was engaged. Her fiancé's some trader up north."
In both cases, girls get the extra 'e' and boys don't-- it's that simple!