Hi Paul,
Did you happen to see the rooster? If he possess blue egg genes there likely a 50% chance your little black pullet will lay green eggs, or 100% if he had two copies (one gets passed on, and the trait is dominant). Blue shell with a brown coat over it produces the green color. There are many genes involved in the production of brown shells so it is most likely that she has at least one hatching from a brown egg. If the rooster came from the same stock as the hen who laid the brown egg, I would expect her to lay an off white/brown egg herself.
Just for your own information: hatching from a brown egg means that the hen that laid the egg was not Ameraucana. She looks similar because of the beard, but chickens can possess beard and other features that look similar to Ameraucanas but if they don't have every trait including 2 copies of the blue egg shell gene they are not necessarily Ameraucana.
Crossing my fingers for you that the rooster had a blue egg gene!
Joey
Here's a photo of coco i took today, but she started becoming very vocal yet, clucking all the time and wandering in and out of the nesting boxes. Hopefully soon, her feathers look beautiful in the sun!