Like a lot of stuff in photography I tend to think of the black sky effect backwards. Sure, Ansel knew how to use a deep red filter against a deep blue sky to darken it. That part is easy. It is the additional darkening in the final gelatin-silver photograph by "burning in" that thrills me.
Making a sky dark by exposing paper to more photons is easy but why is the foreground not darkened too? Because something has been interposed in the exposing light beam to stop it getting to the foreground part of the picture. This process is, I believe, called "dodging". It seems that to "burn in" a sky is effectively the same thing as to "dodge" a foreground.
I once went through the actions of hand dodging a landscape photograph without the negative in the enlarger. Instead I inserted grey filter material of an average density equivalent to the actual negative. Intriguingly I got an image (blurry and out of focus, of course) and realised that it must be a primary photograph of my hands. Because photographic paper is negative acting primary photographs of sillhouetted hands are pale.
Next I put the negative into the enlarger and did the exposing and dodging in the conventional way. The final picture on gelatin-silver paper could be considered as a primary photograph of two things; the negative in the enlarger head and my hands in the light beam.
I have a small Ansel Adams photograph (all I could afford) that is strongly burned (= dodged) and I get goosebumps to realise that the in paler part of it, mixed up with all the image detail, is a primary photograph of Ansel's shadow play in the enlarger beam.
As I say, a strange way of looking at things.