Remove as much of the stain as possible by blotting with a white paper towel, then neutralize with an ammonia solution (1 Tablespoon of clear household ammonia to 1/2 cup of water).
Saturate spot with the ammonia solution using a spray bottle. Do not wet backing of carpet. Blot with a white paper towel to remove excess moisture. All solutions for removing blood must be cool. Apply a small quantity of detergent solution to the spot. (To make the detergent solution mix 1/4 teaspoon of a hand dish washing detergent which does not contain lanolin or bleach with 1 quart of water; example of safe detergents are Dawn or Joy). Use a blotting motion to work the detergent into the affected area. If spot is being removed continue applying detergent and blotting with a white paper towel until spot is removed.
Rinse with tap water using a spray bottle, blot to remove excess moisture. Spray lightly with water, do not blot this time; apply pad of paper towels and brick and allow to dry. If there is still some stain on the carpet and blotting is not removing it, then moisten the tufts in the stained area with 3% hydrogen peroxide. Let stand for one hour. Blot and repeat until carpet is stain free. Light will cause peroxide to change back to water so no rinsing is necessary. Apply pad of paper towels and weight down with the brick.