These dark marks are burn marks from the beater bar itself. most walmart, kmart, and target shelf vacuums from hoover, dyson, and any cheeper vacuum has a hard bristle system. I've noticed even on the high end hoovers as well that they do the same thing. the best way to save your carpets is to see if there is a replacement beater bar that can be substituted for your vacuum. i dont know what style of beater bar that particular one is but check with your local vacuum repair center to see if they have something with softer brushes you can use insted otherwise i would recommend a new vacuum with softer bristles. usually something that is half as stiff as what is already on yur unit. you culd also try lifting the hight setting as a temporary solution.