The usual response to a blued pipe is that the mixture is a bit lean but nowadays you can get the same effect with a repro pipe made out of a slightly lighter gauge of metal or if made of stainless steel but that is more a goldie colour. It can also blue if the pipe doesn't quite fit in the head properly. Does the affected side exhaust pop and bang on the over run?
If none of the above applies it is probably running a bit lean. Don't be confused by the single carb. There is no guarantee you are getting equal filling on each side. It is quite possible you have a slight bias to the right cylinder so it is okay but this means the left hand cylinder is running that much leaner and will blue the pipe.
Once you have checked everything out so you know it is definitely only a bias problem you can cure this by angling the carb very slightly to the affected side. Personally though, I don't think it's worth the bother. Just richen the mixture a smidgin.
Cheers, Bob
