Sorry havent a clue about genetics but I have a couple of photos of my horse as a foal and he was a sort of mink/dun colour. Then photos as a yearling, show him as black.
My filly was born 19 years ago and was the colour of the underside of a mushroom, slightly pink, my vet thought she would either turn grey or liver chestnut. She is black and I mean a definate pure black all over except for a small white 'question mark' on her head and two small white socks. Her mother and father were dark bays.
My ISH x ID colt was born buckskin with a dorsal stripe, bar stripe at the withers and zebra types marks at the top of his legs. Both parents were grey. He gradually changed colour to a darkish bay.
Black foals are born a mushroom/fawn colour and shed to black. Is it not just a regular black foal that has been mistaken due to the tan coloured baby coat?