I wouldn't use anything abrasive, even mildly abrasive. I've heard that Easy-Off oven cleaner works. But, Frau Putzer is too lazy to cook, so I don't have any of the stuff around. I've had good luck with Chevron Techron gasoline treatment, wearing Latex gloves, doing the job outside, and being very careful not to get the stuff on the paint of the bumper.
I still have black "stuff" on the lower half of chrome part of the tips around the lips. But, I'm convinced that's corrosion of the plating, not carbon build-up. I'm confident that the tips will be replaced under warranty on my next visit to the dealership. The car's bee so go so far, that I'm not sure when that will be, likely for new tires. I have access a lift and live 90 miles from "my" dealership. So, I do my own oil changes. I calculated that with my time, gasoline, and wear and tear on the car, those "free" oil changes you get under warranty cost me about $500 each.