This is from an Amy Tenderich column published at DiabetesMine on November 19, 2008:
And now for the Why Blue? And Why a Circle?
I know from my previous research that this symbol was the brainchild of Kari Rosenfeld of the International Diabetes Foundation (IDF) and her daughter Clare back in 2006, when the IDF was campaigning for United Nations Resolution on diabetes (which they got, btw.) The blue color is meant to evoke the color of the sky, and the circle is a symbol of unity.
You can now read the official explanation of the international "Unite for Diabetes" symbol here.
"The colour blue reflects the sky
that unites all nations and is the colour of the United Nations flag.
The blue circle signifies the unity of the global diabetes community in
response to the diabetes pandemic."
And also, from "What's with the blue circle?"
"A simple blue circle because of the circle's frequent occurrence in nature and because myriad cultures over time have used the circle to symbolize life, mother earth and health. The unbreakable unity it represents mirrors the global diabetes community ..."