Yeah the blur effect wasn't really done well on my part as I didn't spend a lot of time on it. The idea behind it though was to match the focus drop off rate of the original to the replaced sky/sunset I put into it. Odds are, with a larger picture, and more time to play with it, that effect could be pulled off more convincingly.
I often go out and take pictures of clouds and sunsets specifically for using them as replacements in other pictures I take where the sky was too flat or boring looking. Careful sky replacement can do amazing things for your images and its fun too, especially when you get it right and no one is the wiser of what you've done!
Below, an example of one that I did recently...
Cheers!
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