Why is there soot on my muffler???

[aetherH3]

I was under my truck the other day taking off my side steps and noticed that there is black soot running straight down the middle of my muffler (the one under the middle of the car), then there is more soot on the driver's side of the rear muffler along the seam... almost looks like there may be a leak or something. Can anyone else check to see if they have this? I'll try to get a pic a little later.