I am having the exact same problem with my new Galaxy S7 - Android 6.0.1.
The screen is basically black when daydream kicks in.
I first found this out because I use Lucid Daydream, which can display the system wallpaper (in my case, YoWindow) and it would actually run for a while (5 mins?) then give an error that YoWindow has stopped. Sometimes it would restart, other times, just black screen. So I shut off Lucid Daydream and used YoWindow directly as the daydream to troubleshoot. I now see the behavior described in this post; I may get 2 seconds of YoWindow, then the whole screen goes black.
Another observation - if I turn off full screen in YoWindow daydream, I get the notification bar showing, but the rest of the screen is black. Full screen=total black. Both will run YoWindow for 2-3 seconds before this happens as it goes to daydream.
Hope this helps for diagnosis...