When we made gallons of blood for Sweeney we used a soap/strawberry syrup/chocolate syrup mixture that washed right out. When desired, this would also allow us to fling the blood as we pulled the razor away from the neck. I've tried numerous mixtures over the years, and nothing works as well as professionally made stage blood, but the Sweeney blood comes in a close second.
"I am ready to disclaim my opinion, even of yesterday, even of 10 minutes ago, because all opinions are relative. One lives in a field of influences, one is influenced by everyone one meets, everything is an exchange of influences, all opinions are derivative. Once you deal a new deck of cards, you've got a new deck of cards."
— Peter Brook