I was watching a cat show on the Animal Planet the other day and saw a segment on the Nebelung. I had never heard of this breed of cat, but it is basically a long haired Russian Blue.
What was interesting was the segment featured a Nebelung that the owners had found stray. They then talked about the history
The Nebelung (NAY-bel-ung) breed began the way so many of our breeds began, a stray cat wandered out of the mist and into someone's home and heart. In the early 1980s a black domestic vagabond-turned-housecat named Terri produced a litter of three- two shorthaired black females and one longhaired male. The father, another anonymous nomad, was thought to have been an Angora mix.
Terri's owner, Cora Cobb of Denver, Colorado, gave one of the female kittens, Elsa, to her son, and in 1984 this shorthaired black domestic produced a litter of kittens that included a blue semi-longhaired male kitten. Subsequently named Siegfried, this kitten became the Nebelung's foundation cat.
Elsa's next litter, fathered by the same shorthaired Russian Blue-type neighborhood Tom, produced a female kitten, Brunhilde, who possessed Siegfried's qualities and characteristics. By that time, Cobb was so taken with Siegfried's beauty and personality that she decided to see if she could preserve his qualities by breeding brother to sister. In May 1986 Siegfried and Brunhilde proved to Cobb that they bred true by producing a litter of three blue longhaired kittens. Cobb named the cat the Nebelung, a German word that means 'creatures of the mist'.
So basically Nebelungs came about originally from stray kitties. So it would stand to reason that that genetic combination could and would and should happen again. I've seen several cats at my local shelter that looked very similar to Jack - in an almost scarey way.