Fake blood is a great addition for Halloween costumes and theater makeup. It needs to look dark and thick, not too runny or sticky. You can make fake blood easily with common, nontoxic household items, such as corn syrup. You can use a commercial stain remover to wash the fake blood out of dark clothing and denim, but it will not work well with white clothing.
Make enough fake blood to really show up on your clothing (and skin, if desired). An ideal amount is a couple of cups.
Mix three parts corn syrup thoroughly with one part water in a large bowl.
Put on gloves if desired; red food coloring can dye skin. Drip a couple of drops of red food coloring into the mixture while stirring continuously. Keep adding more drops of food coloring until the blood is the desired color. Stir for at least 5 minutes to distribute the food coloring evenly.
Add 3 tablespoons of flour to the mixture and stir; this thickens the blood. Keep adding flour, a tablespoon at a time, until it reaches the consistency you desire. Ideally, the blood will be thick but not lumpy, so it can sit on your clothing and absorb without running. Get rid of lumps by stirring vigorously; remove them with a spoon if they will not distribute.
Let the fake blood sit in the bowl for about 15 minutes. Do not place it in the refrigerator. After letting it sit, apply it to your clothes or drip down your skin for the desired effect. Do this with gloves if you wish, so as not to stain your hands. Let it sit on your clothing for at least 10 minutes before touching. It will still be sticky until thoroughly dry, so touch things with caution.
Add a couple of drops of blue food coloring for a more realistic shade after you add the red.