To scream blue murder is to shout loudly and make a huge fuss, sometimes with the implication that the fuss is excessive. But does anyone know why murder should be blue?
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To add to @splicer's answer, the word blue was used in England back in the 17th century to describe someone who looked terrified. This is documented in Francis Grose's 1811 Dictionary of the Vulgar Tongue:
and also recorded in John Ogilvie's Supplement to the Imperial dictionary: English, technological, and scientific.
Which is why it is probably used to scream blue (terrified) murder.
William J. Scott in the Scott's monthly magazine, Volume 4, Issues 1-6 (1867) offers an explanation to why blue is used and not another colour:
Blue Murder is to murder someone of royal, blue, blood.
Murdering a royal is a hard crime to get away with.
To scream blue murder is to denounce someone has murdered a royal or at least accuse them of a heinous crime that they think they had gotten away with.