According to the medieval physiological scheme of the four humours in the human body (melancholy, phlegm, blood and choler), blood was the hot, moist element, so the effect of horror or fear in making the blood cold was to make it unable to fulfill its proper function of supplying the body with vital heat or energy.
RELATED IDIOMS :
freeze the balls off a brass monkey
used in various phrases to refer to extremely cold weather.
Brass monkey comes from the mid 20th-century vulgar slang expression : cold enough to freeze the balls off a brass monkey – the origin of which has been debated. One suggestion relates it to brass trays known as monkeys on which cannon balls were once stowed aboard warships.
1994 - Camping Magazine - David will be doing his best to show you how to keep warm under canvas even if the temperature outside has dipped to brass monkey level.