According to the Online Etymology Dictionary, it is unclear if the origin of the word is Germanic, or Romance. Whatever the case, it was re-introduced into the English language from the French language in the 1600s.
French has two genders: masculine and feminine. It's a hard concept for native English speakers to understand that a noun can have a gender attached to it. However, it is important in the French language.
The form of an adjective in the French language changes depending on the gender of the noun it modifies. If an adjective modifies a feminine noun, an e is normally added to the end of the adjective. Thus, in French the correct form of blond is homme blond for blond man and femme blonde for blonde woman.
Whenever a noun is imported into the English language, it virtually always drops its gender. Blond is a very rare exception. For one reason or another, the word blond has retained a hint of its gender. Thus, if the noun modifies a clearly feminine noun such as woman, the adjective is blonde woman. Otherwise, the correct form is blond, such as blond man.
So there you have it. Amaze your friends as one of the few people who get the correct spelling of the adjective every time.