While most stains, including blood, are easier to remove before they have dried, it is still possible to get dried blood out of a carpet. The most important thing to remember is to use cold water rather than hot. Hot water can set the stain. Before attempting to remove the stain, brush the carpet thoroughly and vacuum up as much of the dried blood as possible. If the carpet is a light color, use peroxide to remove the stain. Be sure to test a hidden area of carpet first to make sure that the peroxide will not lighten the carpet. Spray peroxide on the stain, blot with a dry cloth, and repeat the process until the stain is gone.