NWJS versions 76-85 don't work with worker. This may be the reason of the black screen. Try disabling worker in project properties.
This should be fixed in the upcoming NWJS 86.
In my case, I have the Web worker disabled. And still that weird problem:
It runs perfect first time I start the game from Steam when I just switch on the SteamDeck. But if I exit the game, I have to restart SteamDeck to play again or I get a black screen.
I thought could even be just my Deck, but you can start all games all times you want, so... Don't know. So, so weird...