Mmmm-hmmmm. Fawn/white is what I was thinking too, but there's no way there's any fawn/white genes in there, right? Let's see if I remember... the fawn in f/w is extended black with blue dilution and sex-linked brown dilution, right? Plus the "runner pattern" gene to break it up with white patches. Hmmm... She could have gotten extended black & blue dilution from Finn, but where would the brown dilution come from? And what about the "runner pattern"? Magpie is similar, but they carry a bibbed pattern, I believe, in combination with the runner pattern... I think... sooooo.... let's see... what if William is carrying extended black & blue dilution, masked by the white gene... and the baby received those genes plus the runner pattern but not the bibbed... then you'd have a blue/white runner pattern, but still no brown dilution...
Could the pekin be carrying a brown dilution under the white?? OOoooo! Or, maybe William is carrying it... he came in a box of mixed-color magpies, so he could have had a chocolate magpie parent, which might give him a brown dilution gene, right?
My genetics are a bit rusty... must look this up! You may have a fawn/white magpie x pekin there...