I always do the effect post in photoshop. OPen your shot, then add a 'Selective Colour' adjustment layer. Then in the drop downs select first Cyans, and up the black slider to 100%, then select Blues, and adjust the back to 100%.
You should find blue skys have dipped to a dramatic dark blue. You can adjust the opacity of the selective colour adjustment layer to tweak how much of the effect there is.
If you then Alt+click on the mask icon on the adjustment layer (the plain white oblong), the screen goes white. You can then add a black to white gradient in there. When you click back to the main image layer, the gradient will ramp up the effect from one and of the gradient to the other (depending which way and what angle you added the gradient fill).
Cheers!
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