There are so many different ways you can do edible blood. I prefer this way that is both yummy, without food coloring and super easy to make.
Red Jam (home made or bought) – the important thing is that it is without seeds and fruit bits.
How to: In a pan heat up equal parts sugar and water. Let it boil. Remove it from the stove and add jam a little at a time until you almost have the consistency you want. It should be a bit more lose than you actually want it, because it will become more solid when cold. Let it cool and then use on your cupcake, cake or whatever you want to. If it became too solid you can always reheat it with a little bit of water.