We moved into this house about 6 years ago. It is on the main road in our subdivision. It had a smoky stain running along the edge, just under the vinyl siding. I didn't think much about it, just figured it was dust from the cars, and moisture/dew in the air just drips it there. For the last 6 years, it didn't really change much. 30 days ago we put the house on the market to sell and of course thats when the problems start. We were cleaning up the house, borrowed a neighbors pressure washer, and those stains won't come up. We cleaned the driveway, the back patio, etc, but they won't budge. IN FACT- they are now WORSE. They are no longer dusty grey like they have been for the past five years, but now they are looking more black. I have one idea, and I don't know how much this plays into it. We had a black rubber welcome mat with a filigree design, that is at least 6 years old (it belonged to the previous owner) and it could be deteriorating and the black running into this stain. When I pulled the mat away, the filagree design was there. We were thinking of using the new paint at Lowe's for concrete, but I don't know if the stains will show thru. If there is a cause that needs addressed? Any ideas what it is? On the right side of the porch, it starts about a foot in towards the door and run the length of the front of the house, getting lighter as it nears the left side of the house, but is under both windows. The porch wraps about 2-3 feet around the corner of the house on the left side, the stain doesn't go around the corner. Any ideas, suggestions are greatly appreciated. Please keep in mind we are trying to sell it, not planning on being here any longer, trying to keep expenses to a minimum. Thanks in advance ETA: I live in the Carolina's. The house is built on a slab foundation.