Is it running ok? has it become more economical (crikey) I meant less economical?
Is it louder, could be there is less to scrub the exhaust gasses clean.
Bulie is right, the days of a nice brownish grey exhaust are gone, black is the colour.
Using oil?
Missfiring?
Heavy choking (it has turned a little cooler)
If it's not bust, don't fix it is my advice, mine goes black quickly too.
One thing that could be disturbed by an exhaust fitter, lamda sensors. If you are anywhere nearby pop over and I'll put it on Rovacom (small donations gratefully received).