One trick that I use if I want to bump the sky is using the Tone Eq… You simply hover on the main area of the sky that you want to bump the blue and you increase the curve. The trick is to blend the module in the blue channel then increases in the curve adds blue and decreasing it adds yellow…you can use the adding of yellow to customize a sunset as well…
You need very little adjustment and you can then use opacity to fine tune it but it is quick and easy… .for just the sky you could add a gradient too…