That's like asking why anything is anything. It's a pointless question.
Etymology
The term Krishna in Sanskrit has the literal meaning of "black" or "dark", and is used as a name to describe someone with dark skin. The Brahma Samhita describes Krishna's complexion as being "tinged with the hue of blue clouds",[2] and he is often depicted in paintings with blue or dark-blue skin. In murthis, Krishna is more commonly portrayed as being dark skinned or black. For instance, the Jagannatha (a name meaning: Krishna as 'Lord of the World'), deity at Puri in Orissa shows Krishna as being 'jet black' in colour alongside his brother Balarama, and sister Subhadra, the latter two having much lighter complexions.
So Lord Krishna had high levels of silver in his body?
Does anyone know why he was portrayed as blue?
That's like asking why anything is anything. It's a pointless question.
So is purity or chaos the reason he's blue?
One of the theories is that Vishnu, because of his association with water, is depicted blue; therefore all his incarnations including Krishna, are shown as such. In Hinduism, persons who have depth of character and the capacity to fight evil are depicted as blue skinned. Another theory refers to Vishnu implanting two hairs, one black and the other white in Devaki's womb which miraculously got transferred to Rohini's and as a result, from the black hair Krishna tool birth, with a dark skin, and from the white hair his brother Balarama.
The theory of the blue coloring of Krishna goes to the fact that the Creator has given the maximum of blue to nature i.e. the sky, the oceans, the rivers and lakes. The deity who has the qualities of bravery, manliness, determination, the ability to deal with difficult situations, of stable mind and depth of character is represented as blue colored. Lord Krishna spent his life protecting humanity and destroying evil, hence he is colored blue.
Shiva is also colored blue. Is he also of deep character, defending humanity? I thought he was the destroyer....
Blue has been a sought after color, throughout human societies. It's typically associated with the color of royalty, since it was expensive to manufacture. Virgin Mary is often depicted with a blue coat, for example. That color of blue, prior to the 19th century, ironically, was itself expensive to make, requiring the crushing up of lapis lazuli. Natives decorate their leaders with cloaks of blue feathers or butterfly wings.
It's probably just a relic of blue being associated with kings and stuff.
people can have blue skin color, but it's rare:
http://en.wikipedia.org/wiki/Skin_color
http://www.burlingtonnews.net/bluepeople
I would say that the reason that Krishna is painted BLUE is because Krishna actually had Blue skin color.
With most depictions of skin pigment in religion, tribal or formal religon the depiction of a change in skin color is because that is the actuall skin pigmnet of the divinty.
You will also find that in the dermitologic record there have been people of odd skin color that is not of the normal spectrum of the population.
Some people have been born Black, White, Red, Purple,Green and Yellow. it is not much different from the colors that we see on animals, lizards ect... very rich in color. So you should not be to surprised. some people have multiple skin colors.
Some are actualy born quite regular, blueish green is one of them, red is another frequent one, as well blank white.
I would say that bluish green was the actual skin color of Lord Krishna as based of frequency this occurs in southasia and in india people.
DwayneD.L.Rabon