Mine also had black gunk coming from the iron. It was coming out of the bottom holes and any other area water could leak-I have the DW5080 Focus with 400 holes.
Not only black gunk but water would drain out the bottom. Couldn't figure out where that was coming from. I took off the bottom plate, and I could see up to the water reservoir. There was a black rubber gasket that was popping out where it should have been completely down in order to seal the water in. I took a skinny flat head screwdriver and pushed the rubber gasket back down. It was like this gasket was high enough that heat was getting to it and maybe parts of it were 'steaming' off and the black wasn't really coming out of the holes, but leaking with the water. Once I pushed that gasket back in place, I have never had a problem since. It was like a defect when manufactured. Other than that I love the iron. Make sure to never leave water stored in the iron. Pour it out when the iron is shut off for the day. I have only used bottled water in this iron, and have not had any issues with residue.