Blood boils at approximately the same temperature as water, around 100 degrees centigrade. Human blood is essentially a water solution, made up primarily of water. The salt in the blood, a concentration of approximately 0.9 percent, does little to increase the boiling point of human blood.
Inside the human body, blood does not boil. Human skin is elastic, pressing in on the blood to provide the necessary pressure to keep the body temperature from rising and blood from boiling. Once the heart stops beating or the blood is removed from the body, it is then more susceptible to temperature and can be brought to a boiling temperature.