Maybe warm water helps the still not completely dry blood to stick to the surface? Also how does the salt help the cause? (red wine on a dinner tablecloth) Edit: google says hot water will set the stain. Well, I knew that, what I want to know what exactly happens chemically that makes setting easier with warm water?
Exactly. Drop an egg on your shirt and let it soak in. You can probably get it out with some energetic hand-washing. But put the shirt in the microwave and cook the egg protein, and getting it clean will be a real project.