Grammar lessons

Being a child of the 80s, when grammar was no longer really taught in school, I learned everything I know from reading — so every once in a while, I learn a grammar rule that I had never actually been taught, but had been following instinctively.

Here is an example. It always has bothered me how people use blonde versus blond. I decided a while back that it felt more correct to use blond as an adjective, and blonde as a noun (i.e., "She's a blonde because she has blond hair.").

Last night I finally got fed up and decided to check my AP Stylebook, and this is what I found:

blond, blonde Use blond as a noun for males and
as a adjective for all applications. Use blonde as a noun for
females.

So it seems I was close, as the only thing I didn't know was the gender differentiation.

As a writer, do you ever have instances like this happen to you — where you were never taught the actual rule, but followed it instinctively nonetheless?