We often come across dark stains on the edge of the carpet. This is due to air filtration under the wall. Air is moving under the wall and the carpet is acting as a filter and catching the dirt. The air can moving into the room or could be pushed under the wall if there is positive pressure. Citrusolution can try to remove the stains but our equipment will only reach to within 1/4 or 1/2 inch of the baseboard. You can tell in the right picture we were able to remove a lot of the stains