This is known as carbon. Most cars, diesel and gas as well will send some of this out the tailpipe when they start up. Your's sounds like it has a good straight shot at the pavement. If it's running OK, not to worry.. Merry Christmas.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.