The quality of the chrome plating will determine how soon it will show the effects of heat. The bluing is called the 'heat effected area' in metallurgical terms. A good quality chrome plating will actually be what is called a 'triple chrome plate'. There should be a layer of copper, then nickel then chrome. In recent years they have gotten cheaper and in the U.S. the EPA has come down hard on chrome plating so many shops now just chrome.(many have gone out of business completely) The thinner the plating the quicker it will discolor due to heat. In the case of a catalytic muffler the heat generated, or held, by the metal is higher than a flow-through muffler accounting for the color change you see. No way around it really except to rough up the surface and apply a good high temp paint or professionally applied coating designed for high temperature applications.
Tom