Scream Blue Murder Meaning

(idiomatic) To protest loudly or angrily.

Example: 2003, Isabel Wolff, Out of the Blue, Mira (2003), ISBN 9780778313663, page 144:
  And I was going to shout at him. I was going to scream blue murder. I was going to tell him just what I thought of that.
For more examples of usage of this term, see Citations:scream blue murder.