What is the black line on the carpet around the baseboards of the room?

The black line is caused by air filtration from your HVAC system. As air circulates through your home, it follows the path of least resistance. There is a space between the carpet and baseboards which is the easiest place for air to go when your HVAC system is running. The carpet acts like a filter in these areas, causing the black line around the perimeter of the room. Cleaning the carpet can lessen the appearance of the line, but will not remove the problem 100%.