Too bad it is a Dr. Jekyll/Mr. Hyde tomato.
I’d guess this is a case of Blossom End Rot (BER) but it doesn’t look quite like the pictures. After a bit more searching, I found this article which has an image that matches up pretty well with mine. Looks like I need to find a way to get some more calcium in the garden next year. I’ve tried spraying Rot-Stop on the plants but it seems hard to believe that it could absorb enough through the leaves.