The sky in most of the world, most of the time, isn't blue. It's various shades of grey. A blue aircraft would stick out like a sore thumb against grey skies. This was found out already during WW2 when your thinking was applied by several nations to paint the bellies of their aircraft. Most of them rather quickly switched to various shades of grey (though possibly economics and availability of blue dyes may have been a factor as well) :)
About the only ones who didn't change to grey during WW2 were the US Navy and Marines, they continued painting their aircraft (dark) blue all over into the Korean war, probably because it might make them harder to spot against the blue water of the sea (which often isn't blue either, but that's another story) as well as because it was tradition by then to paint them blue.
But even they switched to grey by the time of the Vietnam war, when the Air Force was still using woodland camouflage on their aircraft.
The British tried various shades of pink and yellow during both WW2 and Desert Shield/Storm to make detection of low flying aircraft against a desert background harder, but they quickly came back from that (probably in part because repainting aircraft each time they are sent to a different theater is expensive and time consuming).