as posted by *rik* A little trick Ive used:

Thread a length of fishing line through a small button with a needle. Carefully punch the needle through the inside of a condom so the button is inside. A drop of latex will seal the tiny hole the needle made.

Fill the condom with blood and seal it.

Attach the condom to a heavy (but small) piece of cardboard with super glue or liquid latex. With a bit of twine, tie this cardboard backing to your actor.

Now score the costume where you want the blood to fly. Thread the fishing line through the costume and set up your shot. When you yank hard on the fishing line, the button will pull out of the condom and the blood will spray.