Reproduction:
-Place a couple of command blocks (or use the "/effect" command)
-Apply the following;
effect <player> blindness 30
effect <player> night_vision 30
If done correctly and activated, the screen will turn completely black. The hotbar, buttons, and GUI will not be affected.
Thank you for your report!
However, this issue is Working as Intended.
The black screen also happens in the non-beta Bedrock version and in Java. A Java developer has confirmed that this is how it's intended to work. If you think this causes a serious problem and want to ask the designers to reconsider, you can do so at this feedback site.
Full Version History – Snapshot Version History – Feature Requests and Suggestions
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki