The background galaxies are from an earlier time in the universe than the foreground galaxy cluster, and thus astronomers see them at younger stages. Young galaxies hold a lot of forming stars, which glow in blue and ultraviolet light due to the high energy that is radiated during star birth. That’s why the warped images of a distant galaxy — i.e., the arcs you mention — lensed by a foreground galaxy cluster appear blue.
Galaxy clusters contain hundreds of galaxies, a lot of hot gas, and even more invisible dark matter. A cluster’s extreme mass warps space-time, and light from more distant objects follows those bends and lands on scientists’ detectors.
Liz Kruesi, Associate Editor