The cooking spray simply acts to prevent the corn syrup from sticking to the tablespoon. The fat prevents the syrup from adhering to the plastic or metal surface. In a pinch, you could also use butter, shortening or even regular cooking oil. Just make sure it's a light coating so you don't add too much oil to the mix.
The blue and green food coloring help darken the red so it doesn't have that bright, fake look. If your blood is too bright, you can add more. Start with just the blue and add green, as well, if you need a more "purple" look.
You want lighter blood for "fresh wounds" and darker blood for those that would have been there longer. Just add the blue or green food coloring, one very tiny drop at a time (a little goes a long way... it's just an accent color). Remember, arterial blood is bright red and veinous blood is dark (almost maroon).
Flour is a thickener. Don't use extra corn syrup to thicken, as it gives the blood a strange, almost sticky (syrupy) look. If you don't have flour, you can add a bit of cornstarch, as well, but use less. (You can also use chocolate sauce; see below.)
Be careful what container you use for the mix. Red food coloring stains, especially if it's left in the container for too long. Use one you don't care about. If you tend to like creepier costumes, just mark it "fake blood" and tuck it away with your Halloween decorations.
Water and chocolate sauce can be used to adjust the thickness of the blood. Water will thin it, while the chocolate will thicken. Only use the chocolate if your blood can stand to be a bit darker. Otherwise, stick with flour or cornstarch.
Adding laundry detergent to the mix is said to make it easier to get it out of clothing. But beware. It may not work on all clothes. Also, you shouldn't use it for blood that's going to be on kids or blood that goes too near your mouth or fingers, as you're likely to ingest some (and even if it's not poisonous in small amounts, it tastes revolting!). Skip this step if you need to keep it fully edible.
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