She's not only white, but the black is diluted blue in some areas.I've also seen this posted a few times over the years, a chicken, usually black or mostly black, gradually growing whiter with each molt like they've suddenly run short of pigment or something. It's also pretty common in Cayuga ducks. Not sure the genetics behind it if it is genetic, but it's not an unheard of phenomenon or anything. I would suspect since it's so common in Cayugas that there would be more answers there if you were wanting to research it more.
I've had a 6yr old hen get a few white feathers, but that can be normal for the Elderly.