Also experiencing the issue (black screen on exit) here quite frequently, but randomly. Seems to affect Dolphin more so than other emulators. I'm using controller automation to exit, but only Alt-Tab brings Big Box back.
Wondering if anyone knows a workaround to ensure the GameStream session ends when you exit Big Box from the menu? Currently it just returns me to the Windows desktop but leaves the session running on the client.
I'm just launching the BigBox.exe executable directly.