Blue was symbolic of the sky and of water. In a cosmic sense, this extended its symbolism to the heavens and of the primeval floods. In both of these cases, blue took on a meaning of life and re-birth. Blue was naturally also a symbol of the Nile and its associated crops, offerings and fertility. The phoenix , which was a symbol of the primeval flood, was patterned on the heron.
Herons naturally have a gray-blue plumage.
However, they were usually portrayed with bright blue feathers to emphasize their association with the waters of the creation. Amon was often shown with a blue face to symbolize his role in the creation of the world. By extension, the pharoahs were sometimes shown with blue faces as well when they became identified with Amon. Baboons, which are not naturally blue, were portrayed as blue. It is not certain why.