Why blue? City historian Tim Samuelson says it echoes the blue stripes on the city flag, which, in turn, represent Chicago's waterways. Water also figures importantly on the city seal, familiar to us in the version created in 1905 by Dr. Bernard Cigrand, a local dentist with a heraldry hobby.
Which explains a few things about the artwork. (The nude kid in the shell, Samuelson says, is young Chicago, the pearl of the Great Lakes.)
Samuelson's history of the seal and flag are on display in "The Flag and Seal Revisited," an exhibit that includes contemporary artists' interpretations of the two official city symbols. It opens this weekend and runs through April 30 at 72 E. Randolph, and it's free. Weekend hours are Saturday 9 AM-6 PM and Sunday 10 AM-6 PM. More hours and information here.