Your description is much the same as many I have heard. Visible in the right light, but not measurable.
What I posit is, the visual effect could be from a heat differential in the barrel which in some way alters the finish OR a momentary stress on the barrel which alters the surface and then returns to original spec via the elasticity of the steel. The bluing unable to maintain its luster as it is stretched and then returned to spec. BTW, we all know that every bullet stresses the barrel, after all, that is how strain gauges work.
Nobody has been able to show that barrel bore dimensions have been changed by shooting monos by actually measuring the bore. I contend that displacement of barrel steel from the inside out - enough to be apparent on the outside surface - must be measurable inside the bore. Nobody has demonstrated that as far as I know.
All that said, I don't choose to shoot monos in any rifle I own but I have it on good authority that a properly designed driving band monolithic bullet OF CORRECT GROOVE DIAMETER is perfectly safe to use even in damascus barreled rifles. Properly designed meaning that the bullet material displaced by the barrel's rifling has a place to flow (into the bullet grooves).