Sovereign 150 with partial blue back
 

Although the blue is closer to the piedmont blue than the Campbell "blue" sovereign 350 example I think this was also probably caused by some kind of chemical exposure.

It does have a spot of blue ink just to the left of the scroll but that could have also turned blue from a chemical exposure.

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I’ve seen this a few times Pat, check out this F. Jones that I have for sale on the BST right now. I also assumed this was from chemical exposure and not factory produced. The front of my Jones card seems a bit faded too, which backs up the chemical exposure theory a bit.

The first time I noticed the blue, I thought my scanner messed up, I had to go physically look at it, haha

Fun Fun

The carboard in the upper right corner of Pat's card appears to be lighter than the rest of the card, which seems to indicate to me that something chemical occurred in that area.

Brian

Another vote for just damage. Green ink can and does turn to blue.

Chemical reaction would be my vote.

I have a T206 American Beauty with a bluish back. I believe it was something that was used to remove it from a scrapbook. Plus under a microscope, nothing else matches that shade of blue. My Blue Old Mill looked like an exact color match to a Polar Bear, until I put it under a microscope and compared other card backs and found an exact match with a Piedmont 150 Blue. You really need to compare the pigment under high magnification.

That T59 Big Run 150 Subjects, Factory 606 came with both green or blue back colors. That's an interesting one.

I would vote some type of chemical also.