Be advised, that you must not rub the blood stain into the fabric when you blot it, doing so will make the stain spread further between the fibres of the clothe. If you can react fast and treat it, the blood stain will not leave any mark at all. If it get dried completely without any treatment, it could stay forever.
I say to use only cold water, because protein-based stains will set on the garment deeper if they are treated with hot water, making it impossible to remove afterwards. You should know also, that you must not any clothes with stains from blood in the dryer unit, otherwise the heat will make the stain irremovable.
In desperate situations, where the blood stain has already set in, a bleach or a lemon juice may help you to get rid at least some port of the stain.
In order to erase a fresh stain from blood, you should soak the garment in cold water mixed with a bit of washing detergent and leave it for a day and the laundry it. What could also help is by dabbing it with a enzymatic cleaner, the end result depends on the type of the fabric. After finishing with the laundry, inspect carefully the clothe if there is any blood remain, you do not want to place it in the dryer. If there still a blood, you could try more.
When the garment with the blood is dried, soak it again in a bucket filled with warm water and put some enzymatic cleaner to make the stain a bit loosen. One or two hours of soaking is enough. Then place it with the next washing cycle. If that doesn't help at all. You can try lastly by using a hydrogen peroxide. As for this cleaning solution, you should test it on a cloth with the same fabric to see if there any burnings will appear later even if it removes the blood stain.