Wouldn't know. I was just repeating what I'd heard. I don't know the real reason. :dunno:
The tan-point pattern is an autosomal recessive. One theory as to why it's forbidden is that the people writing the standard
believed that it would dominate the other colors if allowed, but we really don't know why they didn't want it. It's also not a disqualification in the mother country, it's "highly undesirable" (as is liver). Which effectively means it might as well be a DQ, since a dog couldn't be successfully shown in the UK with a trait so listed. The competition is too stiff.
When the US standard was formulated, I believe I've heard that the AKC wanted there to be a disqualification, so the undesirable colors were listed as a DQ. It makes it more in line with the country of origin anyway, since if they were just "undesirable" here, dogs of those colors would still finish.
Blue is a controversial color, since it is not possible for a blue dog to have the black nose required in the standard. It also wasn't in the original standard, it was added in the 1948 revision. It was very rare until recently, when it's become a fad color, to many people's chagrin. A blue Stafford has never finished a UK championship, since the competition there is too stiff for a dog with such an obvious fault to finish. There are plenty of US blue champions, though. Since it's pretty easy to finish a dog here.