I touched briefly on the idea of blood sponges in a short video from last summer; we were preparing to use them for The Bacchae, but the scene was re-blocked in a way that negated their necessity. A “blood sponge bag” is an extension of that idea. This effect allows you to produce blood on cue with an easily-hidden apparatus.
You need some cling wrap (aka “clear plastic wrap” or “Saran wrap”), thread, blood, and a sponge. Don’t be fooled by the preceding photograph; even though I’m using a fancy natural sponge, cutting a chunk off a regular kitchen sponge will serve you just as well.
Soak the sponge in your blood and wrap it up in the saran wrap. You can fill the saran wrap with extra blood so the sponge is swimming in it if you want.
In lieu of cling wrap, you can also use plastic sandwich bags; your end goal is to create an impermeable membrane which is easily burst by squeezing. Balloons and Ziploc may prove too tough, and paper or fabric will allow the blood to seep through and spoil the surprise.
Tie it all up by wrapping thread around the end. You don’t even need to tie any special knots; just wrapping it a couple dozen times should hold it. You can wrap tightly to put the bag under pressure; this will make it easier to burst.
You can now conceal the completed blood bag on your person until the blood is needed. Just give it a squeeze and out it comes. There is, of course, the possibly noticeable sound of the bag bursting; usually this can be covered through the fight choreography. Because the blood is being held by a sponge, you can speed up or slow down the rate of blood flow by altering the pressure with which you squeeze it.