Literally, Un momento de rubio. I don't know if this would be understood in the same way the English idiom is.
MONA-BLOND
Rubia oscura
Es de cabello rubio (ella)
'Soy' or 'estoy'.
Éste es mi momento.
Translation: Gran momento
Stereotypically, blonds are considered stupid or air-headed. To have a blond moment is to have a moment when you make a stupid mistake or your brain stops working for a minute. You act like an airhead.
Instante de expresión amorosa.
dirty blond doesn't exist in Spanish, instead you could say "Pelo rubio(a) mas o menos" which mens "Blonde hair more or less" Or you could say "Pelo rubio(a) oscuro" which means "dark blond hair" If you mean "blond hair that is not clean", it is "pelo rubio sucio"
Moreno in Spanish
You could say "Llámame cuando tienes un momento."
this is a stupid question