There are several substances that can be effective for removing dried blood, including white vinegar, hydrogen peroxide and ammonia. However, cleaning a fresh blood stain is much easier.
To use white vinegar to remove a small dried blood stain, submerge the stain in a bowlful of vinegar for half an hour, then check to see if it needs more time. For larger stains, lay the item on top of a towel and soak it with vinegar. Again, let it sit for half an hour. Repeat as many times as you need to until the stain is gone, then launder the item as usual.
Hydrogen peroxide can also be used by pouring it onto the stain and letting it sit five to 10 minutes, then blotting. This is generally safe for white clothing or other items, but test colored fabrics first to make sure the peroxide does not remove the color. Light renders hydrogen peroxide ineffective, so wrap the treated item in plastic, and then cover with a towel while it soaks.
Like hydrogen peroxide, ammonia is an effective way to remove stains from whites, but it may ruin colors. With any method, make sure the stain is completely gone before placing the item in the dryer, as the heat can set the stain.