A number of users have reported they are seeing black screens when playing the game, usually when switching resolutions, or toggling between windowed mode and full screen. We've also had reports of similar behavior when trying to use external displays.
We've looked into this issue extensively on our end, and we've found a reproducible case within the engine and have sent it off to the folks at Unity to investigate further.
In the meantime as a workaround, you can generally press Command-F to get yourself into a state where you can save your game, quit, and restart. Pressing Command-F toggles between windowed mode and fullscreen (and back if you press it again).
Pending fix in version: Unknown