The Blue egg gene is dominant. That means that only one blue egg gene is required to produce a blue egg. So if you have cock that has two blue egg gene 100% of the offspring will have blue eggs. If the cock only have one blue egg gene then you start to see problems. You may have 7 hens in the pen with two blue egg genes and one hen with one blue egg gene. If breed to that cock with one blue egg gene 7/8 of the offspring will be from hens that only produce blue egg offspring but 1/8 of the flock will produce 25% of their offspring laying white eggs. So...you start to get every 32nd hen laying white eggs. That is what you got. It is a throw back. Pretty rare but if you want blue eggs it is a big disappointment. If the flock has fewer hens or more than one hen that is only carrying one blue egg gene the probability goes up.