Take a regular japanese back and take a picture side by side
Maybe I didn’t get your point, but if you pulled some blue japanese backs with yellow borders, it will help if you put it side by side with a regular japanese card to see the differences and gather more opinions than just leaving it to imagination
Most likely it’s part of the set and not an error, but again, which set is this?
Those are korean, which have the english back
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Make sure they are actually Japanese 
I believe Korean cards and Chinese cards have yellow borders and the English back!
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