I have a similar issue with black screen in Paramount+. The screen goes black when I started a Paramount+ TV show from the the Roku "What to Watch/Continue Watching" function.
When I start the the Paramount+ channel directly and start the show, no black screens. I have a Paramount+ account that was billed through Roku (I purchased it through the Paramount+ Roku channel, the only way to do it).
I also notice that the Paramount+ channel loads much faster when I open it directly, then if I open it through the Roku "Watch to Watch" function.
Seems like it's a Roku software issue when transferring the TV show information to the Paramount+ channel.