Another spelling rule I will never remember - 

According to the Grammar Girl:

It sounds like a joke, but it's actually a legitimate question: How do you spell "blond"? 

The word comes to English from French, where it has masculine and feminine forms. As an English noun, it kept those two forms; thus, a blond is a fair-haired male, and a blonde is a fair-haired female. 

When you're using the word as an adjective, "blond" is the more common spelling and can be used for males or females; however, "blonde" can also be used to describe a woman or girl with fair hair.