Edge Beta black out

Greg Hejl I have seen this as well, I think it's the same symptom that was attributed to an issue with the browser's GPU process by MS recently. When this happens, hit shift-ESC on your keyboard (shows a browser-specific task manager), sort by task name (click on the word 'Task' in the top left) so things don't move around, single-click the item 'GPU process' and click the 'End process' button in the lower right. This will restart the GPU process and should fix the issue without breaking any open tabs.

Edit: found the post, and tbh they don't mention the screen blackout, just the UI becoming unresponsive. If you see it happen again, try the GPU tip and let us know if it helped. 

This is what MS wrote in that post: "Sometimes the browser will appear to not respond to any user input (clicking or scrolling in webpages doesn’t do anything, hovering over UI doesn’t make it change), but clicking on certain buttons still works (like the … menu). The cause of this is due to an error in the GPU process, and opening the browser task manager (right-click near the window minimize/maximize/close buttons or hit shift+esc on the keyboard) will open a window that will allow you to end the GPU process, which will fix the issue. "