The thing is though, I've seen plenty of faded 1963 Topps cards. Here's an example. It retains the yellow and the green doesn't turn to blue, just to another shade of green:
I haven't found other cards from the 63 set with an all blue box either. Only the Mantle.
It's not unusual that three independent cards owned by who knows how many people since 1963 all have the exact same "fade" with no variation at all? Like for example, none of the three Mantles I've found are only partially blue or even anywhere in between each other. They're the same, even though they've worn independently.