I am afraid low-vision accessibility on Roku has gloomy outlook - but let's see what Roku* have to say.
Couple of years ago, I made an effort of improving accessibility in my iPhone app "Remote for Roku": after reading
this comment under the "Apps for controlling your Roku player" blog entry, i gave it a thought of what can i do to improve app's accessibility. And since i saw Marco Arment said "
VoiceOver accessibility in iOS is extremely easy", i gave it a shot. Turned out there was precious little i had to do - improve button alt. labels - to make VoiceOver feature work well with the app. And so i did. Seems that pirey4@"DVPRemote" has also put some consideration in voiceover - but not the other apps, including "Roku" app by Roku Inc - and that's a reason i am doubtful if RokuCo are concerned with accessibility for the blind.
It is true that if they care, it would be relatively easy for Roku to add "voice-over" feature to the standard types of screens that most channels use. Which they may do - or not do, since it's likely a tiny market segment of vision-impaired people want to use a mostly visual device.
Now, you may be better served by couple of other options: AppleTV has accessibility features (like voice-over) embedded throughout channels - plus if using AirPlay to cast content from iOS device, can also use the iPhone/iPad accessibility features. (Yes, a more closed platform and no custom channels on AppleTV, sorry). Another option is DirecTV - their remoting API allows you to check remotely which channel is on screen and what's on each channel (though AFAIR they removed the access to the full weekly TV guide).