You should try the stain remover that comes with the DRYEL cleaning kit. Try to rub the tip of the bottle into the carpet and blot it up with a towel and it will work well. Then repeat the process until the stain almost goes away.
I love a product for cleaning any carpet product. It is called Folex and can be gotten at Home Depot. It is not just a cleaner but a resurficant. It can also be used in a machine and is sold by the gallon for cleaning machines.
What you have sounds like what is known as a "soil filtration" problem, and may not be related to your candle. If your HVAC return is under those stairs, then it may be sucking air through the carpet close to the walls as it runs. This air contains dust, and over time it is filtered out by the carpeting, leaving a dark band around the walls of the area. This can also be seen under the door of a room that is usually closed off, as the HVAC return constantly draws air from the room under the door.Chris Cole, CR, WLSSentry Construction Company, Inc.Macon, GA