T304/T409 isn't a coated alloy. Usually when you heat it up, it begins to change color. Ever look at the necks on cartridge cases where it looks discolored? That's part of the thermal annealing process (for cases) where it gets heated so it softens up. On exhaust tips, my guess is that the exhaust temperature has gotten hot enough to change the color.

I also noticed the left exhaust giving out more steam/water than the right side exhaust. I use 87 octane gas so all of the carbon simply wipes off. I've seen some S197 exhaust setups where there is sludge-like black tar coating the inside of the exhaust tip/muffler (93 octane, no cats).

 

Well, I've come to the conclusion that it just happens with this metal.

The bronze ring fades when polished. I'll just keep on top of it. They look like new after a good polish though

 

And they sound even better than they look!