If you want fake blood done right, you've got to turn to the professionals. And I'm pretty sure the "I wish this blood were real/yours" look in this man's eyes qualifies him as professional.
He's actually BBC presenter Mark Gatiss, host of a series called A History of Horror. And the goop he mixes up, called Kensington Gore, is just the type of hyperbolic hemoglobin that helps your "slutty [whatever]" costume cross over into "creepy slutty [whatever]" territory. Here's the ingredient list:
2 cups of Golden Syrup
1 cup of Water
10 teaspoons of Red food colouring
A few drops of Blue food colouring
A few drops of Yellow food colouring
10 tablespoons of Corn Flour
Mint flavouring—to taste
Which, you may have noticed, means that it's also 100% edible. Freak. [Buzzfeed]