since when could a black haired guy and black haired woman get a redheaded son? And don't tell me about genes along the family line, I distinctly remember learning that of all the hair color types, black or dark brown is always dominant

Red hair can skip generations*, so as noted above, it's possible that Kathy has red hair in her family line.

* Iit has in my family for example, my father's family has red hair in it though he didn't, my siblings and I don't but I have nephews and nieces with reddish hair, in a couple of cases very VERY red.