Pour 1 tbsp. of cornstarch into a container, followed by 1/2 cup of corn syrup. Mix the ingredients together well with a fork or whisk to break up any clumps of cornstarch.
Stir in 1/8 cup of water. The consistency of your mixture at this point will be the final consistency of your fake blood. Adding more corn syrup can slow down dripping or spattering, whereas adding more water will speed it up.
Add 20 drops or more of red food coloring and stir the solution well. By now the mix should look like fake blood, but not like realistic blood. Mix in anywhere from 1 to 5 drops of blue food coloring to darken the color and make it appear more real.
Test the fake blood to see how well it looks on walls, floors, arms, legs or other places needed for filming so you know how well the blood is received on camera. Make adjustments to the color or viscosity as needed to get the effect you're going for.
Any unused "blood" can be stored by putting the lid tightly over your container and setting it in a cool, dry place.