For a black sky in a monochrome image the color zones module is a nice tool to work on converted black & white images. You can do plenty of subtle changes. But if you like the infrared type black sky look, then the channel mixer module is a more effective tool because you can really separate the colors. The color zones module can’t do that.
To get a good black sky image you need the right images to start with. You need, lot’s of blue sky, and some clouds. This method doesn’t really work on gray skies.
After that you can do it in one Minute:
1. Channel mixer module
2. Convert to Black & White
3. Destination ‘Gray’
4. Pump up ‘Red’
5. Reduce ‘Blue’
6. Control lightness with ‘Green’
More ‘Green’ will make green vegetation look white.
Will post a video later…