Blue is a standard colour according to the
Breed Standard, Grey isn't ;) and brindles are mentioned as any shade of brindle.
They do seem to be becoming very popular with a certain section of society and I'm seeing a lot more of them around. They aren't always of typical conformation though, at least around these parts, but that goes for all colours ;)
IMHO there is a world of difference between the blue Staffs advertised as 'Rare' and the Blue Staffs that turn up in well bred litters ;)
Hopefully someone will be along with info on showing them ;)