I like to use acrylic paint for this kinda application, won't crack or rub off and you can get some pretty good results! You'll need to get the little bottles of acrylic paints any craft store has, get primary red (not dark or light) black and raw umber. The mix is mostly red, add a small amount of black until you get the color you want, a little darker helps on fabric. Add a little of the raw umber to punch up the color a bit, then watter it down to a blood like consistancy. Dab it, flick it, squirt it, use your hands, brushes, sponges to get some realistic spatter and patterns. When it dries it will look more like dried blood, if you want it to be wet, get some Perma Wet and just dab it only over the blood on the fabric. Hope that helps!