Sorry, I don't have an example of a black screen...this has not been occurring for me. But, I am having many, many issues with the VR experience.
1) It is impossible to navigate in an orbit mode. This has been the mainstay the viewing experience for months now. The current orbit experience does not allow me to rotate the model by rotating my head. In fact, it does not allow me to do much at all. Part of the intrigue of the VR system was being able to look at different parts by rotating your head, and I do hope this functionality does not go away.
2) Even in First Person mode, navigation is subpar. I am using an iphone and the Zeiss VR One headset, so to navigate in this mode I have to take the phone out of the headset to move.
3) I have a mesh overlay on many of my models. In VR, this overlay no longer sits over the model but is off by a short distance.
I would kindly suggest that the default orbit mode not go away every time modifications are made to the Sketchfab VR system. When the tried and true functionality of this method goes away, I think many of us (and our clients) are very frustrated. We certainly all understand that this is an iterative process and that improvements are constantly being made, for which we are grateful. But, to never know if the basic components of the system are going to be functional on a day-to-day basis has made Sketchfab almost unusable for me at this point.