The Roku tv settings have a "picture off" feature when watching movies on apps like Netflix and Prime, but this feature doesn't exist when using the Spotify app. Not sure if this is the same for all audio apps. Seems like an oversight since you're more likely to turn off the picture when listening to music. Feature request for the Roku devs.
I posted about this a couple of years ago. It used to work. Although with the latest updates (either Roku app or Spotify... ) This feature no longer exists.
If anything changes, or if I find a solution... I'll be sure to update!
Agreed 100%, would like the option to turn the picture off when Spotify is running. Works fine with Pandora at the moment; same device, etc.