One updated you can enable dark mode by going to Settings General Theme within Calendar and Settings Enable Dark Mode within Keep.
Of course, both the BlackBerry Calendar and BlackBerry Notes app already support dark mode and have for a while now so if you're not in any hurry to switch over to Google's offerings, I understand. Google started pushing out Calendar's dark mode update on May 16 and Keep's began making its way to users on May 20. Depending on that colours you've used that may be a little brighter than you're typically looking for in a dark mode.
Over the last few months, Google's been steadily adding an official dark mode to more and more of its apps. G Suite editions Available to all G Suite editions.