I always wondered what kind of blood is used in movies. I thought it could be from animals or color or least possibly human blood which I am sure will not be legal. I ask because in a lot of movies actors spit blood from their mouths. If it is blood, then how do they do it?
It's called theatrical blood. First of all why theatrical blood?
There are many reasons for substituting for real blood in the film industry, such as ethical and sanitary concerns, and concerns for the emotional well being of the actors. Also, actual blood's tendency to coagulate and solidify quickly make it unsuitable for repeated takes without freshening; the longer-lasting viscosity of stage blood makes it far easier to work with on the set.
What is it? A wide variety of chemicals and natural products can be used. The most common is red food coloring, often inside small balloons coupled with explosive devices called squibs. Alfred Hitchcock famously used Bosco Chocolate Syrup as fake blood in his 1960 thriller Psycho. Since the film was in black-and-white, the color was less important than the consistency. Tomato ketchup is also a common alternative.
Fake blood can be made in many different ways. One of the cheapest and easiest can be made with varying recipes including corn syrup and food dye. For example, in the movie The Evil Dead the makers used a mixture consisting of corn syrup, food colouring, and instant coffee.
Chocolate syrup is also used with red food colour to achieve correct shade of blood.
Here is a history of Fake blood usage.
And if you are really interested in step by step method of making fake blood then refer wikihow.com, where fake blood making is explained in 14 easy steps.
Fake blood is also referred as Theatrical blood or Kensington Gore (after the street of that name in London).