I have many decades of Zz growing experience, but have only been growing 'Raven' for about 5 years. It seems to needs more light than the regular forms, if you want strong black leaves.
Additionally, I don't water my Zz's at all in the winter as it would just result in soft floppy growths. A well-established plant has no problem with a six month drought.
If repotting is needed (and I wait until they practically break the pot), do it in summer.
