To my eye that looks like a regular base card. I just don't see how it's anything else, let alone a 1/1 Mirror Black.
The sides appear to have a mirror finish rather than the matte gray. That comes through on the picture, I don't think that can be done with just camera angle and filters. Perhaps a different camera angle could have brought out more of the shine than the scanner.
These were not labeled as mirror blacks. There are a few older posts on here and images on BaseballCardPedia, TCDB which have similar looking cards. Will try to post images.
I feel it was a great purchase by the buyer!