Since this day starts a new season, and I like to sandbag, I spent a great deal of time entering and exiting the hero screen (may be important to hunt down the bug) to improve skills and equipment.
The last time I tried to exit the hero screen, I was greeted with this dark screen.
I thought long and hard about whether it would be better to save and exit, or just exit. It let me manually save, so I did. I then exited.
No damage done, near as I can tell. Everything was saved, and the interface is working properly once I reloaded.
I was using an iPad when it happened.