How much blue is how much you can stand. I've got some very blue sections in my exhaust pipe. I don't care. How hard were you riding it in twenty minutes? That was bluing seemingly rather quickly, but there are a lot of considerations, so it may not be of any concern. But you have a single section pipe, so it would be inclined to blue pretty quickly, but on the other hand, you may want to take a look at your plug and check your mixture.
Bare