In his arguments against Charles Darnay, who is accused of treason, the Attorney General calls for Darnay's death:
When the Attorney-General ceased, a buzz arose in the court as if a cloud of great blue-flies were swarming about the prisoner, in anticipation of what he was soon to become.
Dickens describes the crowd of the Old Bailey as having "an insensate brutality and ferocity worthy of Abyssinia or Ashantee." However, when, through the genius of Sydney Carton, the witnesses' identification of Darnay as the passenger on the Dover mail stage becomes dubious and he is acquitted, the "blue-flies" who have buzzed again at the testimony of one Roger Cly, now grow quiet. But, when Miss Lucie Manette tearfully says,“I may not repay him by doing him harm to-day” because she does not wish to implicate Darnay, the flies "buzz" again in hopes of finding another victim. But, after Mr. Stryver, with help from his associate Sydney Carton, creates doubt in the mind of the jury of whether the witnesses did, in fact, recognize Darnay or perhaps some other man since Carton and Darnay both resemble each other,
And now the jury turned to consider, and the great flies swarmed again.
After the jury finally returns, Charles Darnay is acquitted and
... the crowd came pouring out with a vehemence that nearly took him off his legs, and a loud buzz swept into the street as if the baffled blue-flies were dispersing in search of other carrion.
"Blue-flies" is also an allusion to Beelzebub, the devil, who is often represented by such insects of this color. In the courtroom where Darnay is on trial for treason, there is an evil that circulates the room. This evil connotes the foreboding of another evil: that of the revolutionaries who swarm in hopes of the blood of the aristocrats later in the novel.
Charles Dickens began publishing installments of A Tale of Two Cities in 1859. The forty-five chapters that make up his novel are spread out over three "books."
The action of Book 2 takes place five years after the conclusion of the novel's first book. The trial of French aristocrat Charles Darnay, who is being accused of treason against England, opens the second book. The penalty for treason was death and the blood-thirsty crowd is anxious to see Darnay (or anyone for that matter) be convicted. Unfortunately for the crowd, Darnay is found not guilty of the charges against him.
When the crowd realizes that Darnay has been acquitted and that the trial is over, the crowd pours out of the courtroom. Dickens describes them as being like flies in search of some other dead animal to light upon:
a loud buzz swept into the street as if the baffled blue-flies were dispersing in search of other carrion.