Blood is difficult to remove from carpeting because its hemoglobin bonds to the fibers of the carpet in the presence of oxygen. It is especially difficult to remove if it has dried or been subjected to heat.
If blood is spilled on carpeting, clean it up as quickly as possible. The more quickly it is removed, the less likely it is to make a stain. Blot up the blood using a clean white cloth. Repeat until no more blood appears on the cloth, using a clean patch of the cloth each time, and spraying the spot with cold water in between blottings. Do not rub at the blood while blotting, as this works it into the carpet fibers. Once the carpet is free of any visible stain, cover the damp spot with a clean towel or stack of paper towels, and put a weight on top for several hours to soak up any remaining moisture, then allow the spot to air dry.
If a stain remains after blotting, the next step is to sponge the stain with a mixture of 1 tablespoon of liquid dishwashing detergent to 2 cups of water, or 1 teaspoon of a mild non-alkali laundry detergent to 1/2 cup of water, and blot repeatedly until no more liquid is absorbed into the blotting cloth.
If the stain is still there, the next step depends on the type of carpet. If the carpet is synthetic, blot with a mixture of 1 tablespoon of ammonia to 1/2 cup of water. If the carpet is wool, blot with a mix of 1 part vinegar to 2 parts water. If the carpet contains silk, have it professionally cleaned.