This is all very interesting. Kiwi only has a teeny tiny splash of blue on his front but he does seem to gain a few new blue feathers each molt and doesn't appear to be loosing any blue. My mom's DYH has had her yellow spread throughout her life and continues doing so the older she gets. Now her neck is becoming spotted with yellow and will presumably fill in over time like her head did.
Is it possible in the old picture of Gonzo a flash on the camera gave him a bit of a blue "sheen"? In the right lighting Kiwi's green plumage can look to have a blue sheen to it, especially his neck and chest area. From my understanding, they see farther into the light spectrum than us and their plumage actually contains colors our eyes cannot perceive but prospective mates of their own kind can

. Perhaps with the right circumstances, a camera can help us "see" a bit of those extra colors too!