You can make glow in the dark blood for a Halloween costume or just because you want to decorate yourself with glowing blood. Here are several recipes for non-toxic glow in the dark blood.
Glowing Alien or Radioactive-Looking Blood
Mix equal parts white corn syrup and clear non-toxic school glue (the kind that washes off with water). Add liquid from a highlighter or some glow powder or paint from a craft store.
If you use liquid from a highlighter, you’ll get blood that will glow the color of the highlighter under black light. Not every highlighter glows, so test it before using it. If you use glow powder or paint from a craft store, your glow in the dark blood will glow when the lights are off, providing you “charged” the blood up by shining a bright light on it first. Glow powder typically glows yellowish green.
I had great luck with this glowing blood. It was very bright and washed off easily with warm water. It likely will stain clothes, so watch out for that.
Glowing Bright Blue Blood
Petroleum jelly, laundry detergent, and tonic water glow bright blue under a black light. The easiest way to make blue blood that glows under a black light is to drizzle liquid laundry detergent on yourself (which is not edible, so avoid getting this into your mouth). Tonic water with corn syrup will make a thin blood. You can tint the blood with food coloring, if you like. Petroleum jelly can be applied as a thick, non-dripping blood.
Glowing Red Fake Blood
You can make red blood that glows red under a black light by either mixing pink highlighter liquid in with any of the fake blood recipes or by adding chlorophyll to a recipe. You may be able to purchase chlorophyll, which fluoresces red in ultraviolet light, or you can prepare it yourself by grinding spinach or Swiss chard with a small amount of alcohol (e.g., vodka) and pouring it through a coffee filter to get chlorophyll extract (use the part that stays on the filter, not the liquid).