My suggestion would be to have several costumes for each actor. Wear dark clothing with your only light pieces being where the blood needs to be seen, then change those (light) shirts/pants/skirts etc between each show. It will make things slightly more manageable if you use or make fairly thick blood rather than anything really thin that spreads quickly.

Your show is probably over and done with, but I did have an idea.

I don't know how "real" you are going, but perhaps you could simply represent the blood symbolically.

A good example of this can be taken from the musical and movie, "Chicago" in which red silky fabric represents the blood during the “He Had It Coming” scene.

Not knowing anything about your show, perhaps the "victim," when they are shot, could pull a red scarf or cord from under their shirt and drape it across their shoulders.