Some tvs give you the option of shutting off the video while continuing to play the video. I have a low end JVC in the guest room with this option.
Without such an option, the only possibility I've come up with for this is to install and use a black screen saver on your Roku. Here's a link to several of them: https://channelstore.roku.com/search/black+screen
Roku Community Streaming Expert
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X