That's surprising, I had the same thing happen to me with my 2013 Spring chicks.
I bred Mona a visual cobalt blue female with Carioca, a dark green male and they had 3 chicks one of them was a recessive pied just like yours only mine is yellow. You can see Peppy and the parents on my signature.
Your budgie couple, just like mine is hiding the recessive pied gene. You were very lucky to have this happen with just the one chick!