So you are going along in your day and you cut yourself. Maybe you nick yourself shaving or slice your finger in the kitchen or get a bloody nose. Unfortunately you get a little blood on your jeans or your favorite shirt.
Don't worry. Spit on it. That's right your own spit will get out your own blood. So if the blood hasn't dried give it a little spit.
Every time I tell an actor or a fellow costume person this they are amazed at how well it works.
I had a wardrobe friend who had kids and after she had her kids she had super spit and could get out not only her own spit but her children's spit as well.