Shooting IR for the last 2 years I've come to conclusion that the blackness of the skies depends on season, atmospheric conditions of a given day, time of the day (Sun overhead to reduce atmospheric scattering or the red in the skies as blue color goes bye bye first) and CPL when the first factors don't align to your liking.
Extra blue skies will be extra dark in IR
I agree the "golden hour" of infrared film is high noon, when the foliage emits a lot of infrared red while the sky can be deep blue. Thus accentuate the infrared look.
On the other hand, I often like to have interesting clouds in the sky. Ideally the foliage is still in direct sunlight. And I also like the dark clouds shortly before a storm, which is even more dramatic in infrared.