When did the phrase "in cold blood" first originate?

Answer 1

"A person whose blood is cold or cool, so detached or uninvolved."

It was first recorded in Joseph Addison’s Spectator in 1711.

The phrase is now usually taken to refer to some act that might look like an act of passion but which was actually done with cool deliberation.

http://www.worldwidewords.org/qa/qa-inc1.htm

Answer 2

It became a famous expression and book when Truman Capote wrote a novel "In Cold Blood" about the murder of a family of four in a small, quiet town in Kansas (@1960's). Capote spent many hours in prison visiting the killers (2 men) and learning about their lives and details of this gruesome event.

I highly recommend the book and the movie, "Capote." He never wrote another book again; this one changed him.