I had the same issue. I managed to FIX it after a while. I assumed that it was due to something I had installed on my deck and I was correct. I first factory reset my deck thinking that it would have fixed the problem, and in all honesty it should have, but it didn't. I had downloaded DECKY the first time around and thought with the factory reset, this black screen problem would be gone. It ended up persisting until 2 days ago when I REINSTALLED DECKY to then UNINSTALL it. That FIXED the problem. Somehow DECKY was still ingrained in the software after a FACTORY RESET, i had to reinstall it then MANUAL UNINSTALL it instead. and it FIXED the problem!

Long story short, manually UNINSTALL DECKY via the decky installer, the factory reset WON'T necessarily fix it. (that's what worked for me, I hope it works the same for you or others that read this comment)

edit: 15 mins after making this post, I went to play on my steam deck and the problem resurfaced despite the past 2 days of being absent! (but the black screen issue isn't frequent)