This article says (bottom of page 10) that postal collection boxes were ordered to be repainted after the passage of the Postal Service Reorganization Act. Why blue?
It's actually in your document.
The next year, Loewy/Snaith suggested painting collection boxes one color instead of three, recommending a sky-blue color. Sky-blue paint was tested on a few trucks, but the color failed to impress postal officials. In 1968, the Department began testing dark blue collection boxes in four cities. To make them easier to find at night, the boxes had reflective decals featuring the words "U.S. MAIL" and the Department's post rider seal. By April 1969, Department officials settled on the dark blue color.