In Photoshop I use the LEVELS Control to "Tweak" the sky a bit on the Dark Side to make SURE that the Sky is TOTALLY "0-0-0" Black. I use the Color Sampler Tool to check. Sometimes a bit of SUN GLARE will make a corner slightly GRAYER (less than 0-0-0) so I select that area and fill it with 0-0-0 Black so that its all even.

BTW- If you copy these its o.k.- but you will NOT get
the same results as using a much HIGHER REZ Cloud Mask.
As stated above- I render these at a minimum of 2048 x
2048 for best results...