No type of fake blood is 100-percent stain proof. Even those made from gentle formulas may leave a mark on delicate fabrics and you shouldn't wear anything with fake blood that you don't want to see stained. Having said that, there are a number of recipes for fake blood that are much gentler on fabrics than others and can reduce or eliminate the risk of stains on your skin or clothes.
Things You'll Need
2 tsp arrowroot
Red and brown powdered tempera paint
Add 2 tsp. of arrowroot to 1 cup of hot water. Stir the mixture until the arrowroot is completely blended and has reached the thickness you desire. (You can vary the proportions to change the thickness and consistency of the blood.)
Stir in a measure of red tempera paint , 1/2 tsp. at a time, until the mixture attains the level of redness required. At this stage, the blood will look bright red, like the blood found in a Hammer horror film or a Sergio Leone spaghetti western.
Cut the mixture with a tsp or two of brown tempera paint, adding it 1/4 tsp at a time until the blood reaches the desired color.
Add water as desired to thin the blood out. Add the water slowly so that you can gauge the thickness: if you use too much, the blood will be too thin and you'll need to start again from scratch.
Arrowroot can be found in most specialty health food shops. It's often used to help thicken foods such as pie glazes and fruit fillings.
Make sure the paint you use is non-toxic. Use painted designed for use by children--check the package to make sure it's labeled "non-staining." Even non-staining fake blood runs a certain risk of staining clothes. Use the fake blood in small amounts if possible, and don't concentrate it in too many places on your clothes. Wash your clothes as soon as possible, using concentrated detergent.