Not sure if this has been clarified, but I thought I would offer my two cents on it.
I recall there being some question as to whether the 1958 Aaron card with the blue back was sun bleached or just colored weakly. I just picked up a card which may clarify this...
If you notice, there is a green area on the background which has some yellow color. It helps that the yellow is slightly out of register also. One can see that the yellow plate was placed partially on the image.
Because there is a partial yellow overlay on the background and the rest is left intentionally blank, it appears there is or should be a blue background variation which is accepted in the hobby. This shows that the blue background card was created by not having a yellow overlay. That the blue background is not a result of sun fade.
Therefore, the missing yellow plate ought to be a variation, not a poorly kept card.
AARON.jpg