Waterman, don't worry! The silencer on my 2014 B5 looks exactly the same and has done so from a very low mileage.
The heat shield covers up the discolouration, so I don't care. As Vince says, the heat shield makes the silencer run hotter because the air can't get to it, hence the blueing.
Having looked at your photo, the same is true of yours. Put the heat shield back on and it'll look fine.