Soak your shirt in COLD water immediately, before the blood dries. After it soaks for a half hour or so, rub in a paste of Adolph's meat tenderizer (unspiced) and laundry detergent. Throw the shirt in the washing machine (COLD cycle,) and the stain will probably come right out. Check to make sure all of the stain has been removed before putting the shirt in the dryer, since heat tends to set a bloodstain permanently.
I have used this method many times with my sons' clothes, and only once did it fail, when the stain was already dried in.
The reason the meat tenderizer works is that it contains enzymes that dissolve animal proteins. This makes the blood bond with the fabric less tenaciously, and it rinses right out.