The logo is given blue color as anti to red that is wars. designed by oliver lundquist. there are some other reasons which are not mentioned.
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The flag contains shining and pleasing blue colored background.  The blue color is treated in the west as a vibrant colour, meaning love and opposite to redness. Red color is a symbol of war.  So blue color is for representing that UNO wants world peace.  Also the white color is the official color of the UN and is used along with blue for various clothes, articles and products related to UN.

In the center we have the azimutal projection of the Earth as viewed from the top of north pole with the international date line at the center.  This shows that UNO represents and cares for all countries and continents.

There are two Olive plant branches below the (azimutal projection) globe. The represent peace and spanning the entire world.

There are five concentric circles drawn which represent the latitudes.  The number five was probably to indicate five continents, when the flag was designed in 1946.

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