
My screen became entirely black other than the top row of buttons (concede, leave game, etc.). Only the play window was blanked, the other windows (chat, deck builder, etc.) were normal.
Screenshot attached,
I've seen this happen locally when the game server crashes. Can you remember what you were doing just before the screen went black?
I've had a screen blanked problem as well.
My opponent had experienced some connection issues earlier but could eventually get back into the game. He also told me that there were several times when he tried to rejoin the game and he just got a black screen.
Then, a few rounds later, I suddenly had a black screen instead of the playing surface. Tried to log in from another browser and rejoin. Tried to refresh my main window, left the game, tried to join again but it would not work, because by then my opponent played themselves.
I've included a screen grab for demonstration purposes:

I also saw another bug report having this problem - Connection aborted #3172
I've also saved our conversation log but as far as I can tell the blanking always happened on the other persons turn...
This usually indicates that the Javascript in the client is crashing. If you happen to see it again, it would be very helpful to get a copy of the browser console text.
Most helpful comment
This usually indicates that the Javascript in the client is crashing. If you happen to see it again, it would be very helpful to get a copy of the browser console text.