Have you tried searching the word "Roku" in the App Store? I found an app specifically made by Roku. I cannot attest to its usability with VoiceOver, but it looks like a fairly simple menu system.
Many in the app store actually. Roku has their own, and there's DVRRemote, amongst others. The problem is that they cannot provide feedback to what the Roku has showing on the screen, and the Roku is menu driven. Meaning, after you select a "channel" you then get a list of show types, and/or shows, then a listing of specific show times. This only shows on the TV screen, it does not show back to the app. So for a blind VoiceOver user, the apps really do not help unless you have some other way to read the TV screen. In which case you just as well use the Roku remote as often as not. I encourage you to email ROku, maybe if htey hear enough complaints they will implement more functionality into the Roku API, as it seems this is a very possible thing to do if they wanted to...
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App Store
Roku apps