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No. How the **** could that even happen? Unless the case interferes with the flash, there's no such thing as making the photos darker.
 
I've heard about some cases making some photos appear washed out when using the flash. I think that's why many have a black ring around the camera cut out. Not sure nowadays with the wider cutouts on the 7 Plus cases.

I've never heard of pictures actually appearing darker though.
 
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