This should help:
Edit:
I saw a tip on Reddit which suggested that the cause for the black/white screen is generating new shaders after the patch and the solution is just to wait for 5 minutes or so until they are generated.
I can confirm that worked for me on RX6600 card: I removed the DX11 option and started WoW, WoW went to not-responding black screen, I alt-tabbed to do something else and eventually WoW started normally. After that WoW starts instantly when using DX12.