Hi Jan,
Billy is gorgeous! You are so lucky!
Although my Tilly is jet black, her mum and her litter sister are both black and tan. I think (and I'm sure someone more experienced will correct me if I'm wrong) that the tan is a recessive gene in some solid colours and so is subject to the usual statistical rules of genetics. Also, I think you can only tell if a dog carries the tan gene by breeding from them - which may also explain their rarity! I think the genetics of colour in the cocker spaniel is a fascinating subject. Tilly's dad was golden, so in her litter there was a lovely mixture of 2 black, 2 golden and 1 black and tan puppy.
I have seen a few black and tan cockers doing well in the show-ring (including Tilly's lovely sister).
I think the black and tans are gorgeous and I am sure you will enjoy many happy years with the cuddly Billy!
Sue