This scene has the look of a photo made with the camera deliberately set to Tungsten white balance, and an orange-ish CTO gel placed over an electronic flash---that orange light will be rendered as white light, due to the off-kilter white balance, and the natural light from the sky will go excessively blue. Using that method, it's possible to turn an ugly gray sky into a blue sky. I am guessing that this photo was made in a similar fashion; perhaps the OP will enlighten us?