One must always use cold water to wash blood stains out of fabrics. Hot water sets a protein stain, such as blood, and makes it more difficult, if not impossible, to remove later.
Prior to washing the garment, it is best to soak the stain in cold water, rubbing it from time to time to get rid of the blood. If the blood has dried, scraping the dried part off of the fabric first is helpful. It is also advisable to add some laundry detergent or a pre-wash product effective at removing protein stains to the cold rinse water. With many fabrics, pouring hydrogen peroxide directly on the stain rids the garment of the bloodstain completely.