From Lost Fables Wiki
Tainted blood, depending on the looks of the halfblooded, is either extremely pale or white, or such a dark red that it is nearly black. Whichever it is, the blood has some unusual properties, clumping up into disgusting little squirming knots when it is spilled onto the ground and rapidly diluting into any fluid it is put into, like food dye. The blood, despite its disgusting features, is actually extremely nutritious.
Like most creatures, the Blooded are very protective of their vital bodily fluids, and are not typically easily persuaded to be bled. Nevertheless, if you manage to convince one to give you some of its blood, they can safely lose up to two liters per age, but no more than 500mL in a day. A single dose is one liter.
Tainted blood is a supremely useful ingredient in almost any ritual to summon a demonic entity, in addition to its alchemical effects. When made into an injection, tainted blood makes a creature about 50% stronger and faster, and more capable of fighting off illness for 6 rounds, though it is poisonous when made into a potion, and ineffective when used otherwise. It cannot be mixed with other ingredients.