Yeah, I kept thinking about this one all day. All I came up with is that one would have to be dom. White and one recessive white and one would be black under the white. But that is pretty far fetched, however it could happen as I have dom white and recessive white here. More than likely, I would say Toni has guessed right on this one however. Just doesn't seem possible and isn't possible with two rec. white birds. Unless you are like cy and have splash red (also called rec. white) and true recessive white. Which will cross together if pure strands and produce colored birds. Or so Cy said his did. And that is how we ended up with bbreds.
I am relatively certain that no other bird in my flocks have bred to my hen. She has been isolated with the white roo since I have had them, and I have had them for nearly 2 months or more if I remember right. Not only that, but the first 6 or 7 eggs she laid were unfertile. This was the first of my fertile phoenix eggs. I don't know if the chick is "pure black", all I know is that the hair is completely black so far. And I don't know the genes of my white birds, so I don't know if they are "pure white".
On a sad note, my phoenix chick died this morning. There were some complications in the hatch that I didn't see when I posted a few days ago, and I don't think it absorbed all of the yolk sack before it was out of the shell. Broke my heart, but at least I got to see that it wasn't going to be white. I have more in the incubator so we'll see what happens with them.