Years ago, black exhaust tips usually mean that your engine is running rich, but that is not the case today.
I have a Mercedes with chrome exhaust tips and they have gotten black since the day I bought it new. I also have a new turbocharged 20016 Hyundai Tucson Sport with chrome exhaust tips and they have black smudges on them in only 3K miles. Sometimes the black smudges are more towards the bottom of the exhaust tip because condensation drips out of the exhaust system. That condensation is mixed with the black carbon particles it picked up in the exhaust pipe
Both vehicles get excellent fuel mileage (better than EPA estimates), great performance, and not running rich.
All gasoline vehicles emit some carbon and have black smudges on the exhaust tips, it's just the chrome exhaust tips that makes the black smudges more noticable. Some good chrome polish should reduce the deposits and make the black smudges easier to remove.
...Rich