The blue-flies buzzed again,
Describing the crowd witnessing the court trial as "blue flies" is such an interesing symbol! Flies usually represent decay or filth, since flies appear around such atmospheres. If the crowds are blue flies, then Dickens is saying they are not comprehendable. They buzz and each looks the same. It's a mindless swarm bent on and driven by the possibility of witnessing a brutal death. They are easily swayed with no individual thought for themselves. Nicely done.
