In many homes with light carpeting you will find black edges on the carpet along skirting and near door frames. This is caused by a build up of dust and can be from ventilation where the wall meets the floor. The only way to remove this when its already happened is by carpet washing. To help prevent buildup you need to brush around the edges of your carpet periodically. So called 'edge to edge' vacuum cleaners are not sufficient so vacuum the edges using the manual brush/crevice every time you vacuum.