They tell me there are several things you can do in photoshop to help that sky... A graduated filter comes to mind, but im not into digital. I'm sure some of the digital people can make you a good suggesting though.,
Crank up contrast.
Select the white sky.
Adjust Hue/Saturation to appropiate shade of blue.
New layer withe sky still selected.
Render clouds.
Change opacity to around 40%.
Vualah.
Yes, a NATURAL blue sky would help this photo a lot, since photos of hot air balloons are good in themselves and fun to look at and usually very colourful and ... they always make me wish I could be in a hot air balloon myself one day ...!