note: use the cheap liquid food colouring and not the expensive gel concentrate that I have pictured here. The gel colour stains very easily while the regular red liquid food colouring has washed off of all my costumes in the past.
Pour about a cup of corn syrup into a jar. Squeeze in some chocolate syrup (or add a tsp or two of coco powder) and then stir in red food colouring until the mixture is a deep red that is still a bit transparent like blood.
Next you can pour it on your neck and costume for a sticky blood effect. Or carry it around in a water bottle with you so that you can 'bleed' from your mouth when needed. (I kept these pictures as links to my flickr rather than posting them directly to the blog because they are the opposite of appetizing) You can see the edible fake blood in use here , here, and here