Pour 2 parts Karo syrup and 1 part chocolate syrup and 1 part clear dish soap in a large plastic bottle. A water bottle or squirt bottle will do.
Add red food coloring followed by some blue to achieve the desired color of blood. Realistic blood is reddish-brown. Screw on cap tightly.
Shake the bottle, mixing all ingredients until the texture is smooth. This version of stage blood is very sticky and likely to stain. Add food coloring to adjust the color if necessary.
For edible stage blood, substitute peanut butter for the chocolate syrup and dish soap. Just a spoonful of peanut butter for a bottle of Karo syrup and some food coloring will suffice. Heat the bottle of syrup so that it will mix more easily with the peanut butter.
Add a capful of photographer's wetting agent, like Kodak Photo Flo, per cup of fake blood if you want blood that saturates clothes and fabric more realistically. This version should not be used near the actors' mouths.