The blue colour in dogs is just a dilution of black, just a genetic fault.
The blue in the apbt didn't come from the sbt and vice versa, like I said it's just a genetic fault but some people obviously liked the colour and bred the dogs with the fault as so to produce more.
It is possible that you could breed game bred dogs that are not blue and end up with a blue pup, but in general a lot of blue dogs tend to be curs, apparently.