When trying to watch some streaming services on my Roku tv the screen turns black and the audio still works.
Did your Roku recently get updated to Roku version 11.5?
Roku operating software version is in the process of updating from version 11.0 to 11.5, individual Roku installation by individual Roku installation, with a schedule known only to Roku. If you have multiple Rokus, they may not become eligible for the update at the same time, even if they are the same Roku model.
Roku menu path Settings > System > System update will show you your current Roku version number. Down in the lower left corner of this screen it will also show you when your last update was installed.
A new release sometimes creates problems that must be addressed. If you are having a new problem with your Roku, does its beginning coincide with a 11.5 installation date?
In past major Roku updates, I've found that updating the Roku software may not automatically update all channel apps that have new versions to accommodate the Roku update. Manually running an additional update check as soon as you receive the Roku update can make sure you have the latest versions of all your channel apps and prevent many update-related problems (Settings > System > Power if it exists on your Roku model > System update > Check now).
Despite having had access to the Roku update as it was being developed, some channels may take a few more days for their developers to issue needed updates, so a manual update check as mentioned above to catch any more recent updates is always a good first step when running into specific channel problems.