Skies are difficult because they change colour according to the time of day and all times according to distance upward from the horizon. I've gotten natural looking results for a daylight sky valid from about 10 in the morning till 3 in the afternoon usinf Lab values of 75 -5 -24 for the bright portion and 60 -5 -25 for the darker portion. To blend them smoothly, you want the brighter sky closer to the horizon and the deeper tone further away. Put them both on separate layers, add a layer mask and use a black and white gradient on the layer mask to blend them gradually the one into the other so that it looks like a natural progression.