When a person (usually a blonde) does something so ridiculously stupid and obvious a 7 year old could figure out
The reason why it's called a "blonde" moment is because of the acts that blondes do. It also happens to blondes the most or girly girls.
<Chelsea> Jadden concentrate on your work!
<Jadden> Isn't that stuff in my orange juice?
<Chelsea> Oh My God! you just had a Blond Moment!