That's a funny story. Thanks!
When I was a kid we had a white kitten that had dark greyish brownish stripes on the top of his head to face. Once the cat fully grew the markings completly disappeared. It was so unbelievable. Apparently most white cats are not really white but they "Pretend to be white"

do it really well. They are of some other color but the white somehow mask this. I don't know the ins and outs of it.
Perhaps you have the same situation with tabby pattern dominating the black fur?