If you don't mind a bit more gooey blood use corn syrup, red food coloring and a wee little bit of hershey's chocolate syrup.... used in the movies for blood too!
yes, you could us a glaze, using powder sugar, milk,etc. Karenscookies.net has a good tasting one for her cookies. I make it when i make cookies. hth
I used red gel-dyed piping gel for the blood on my sword cake (in my photos.) Although this might be too runny for this purpose. Americolor SuperRed dye will make a nice deep blood red. Maybe just dye your buttercream red and thin it out with a little water and drip it on. HTH.
Thanks ladies for your great ideas!!!