Black screen on Apple TV to hen trying to mirror app from iPhone x

Use AirPlay to stream video or mirror your device’s screen - Apple Support

If AirPlay or screen mirroring isn’t working on your device - Apple Support

Please do the checklist in the second link.

If you can AirPlay the iPhone home screen, then AirPlay mirroring itself is working, and any issues would be app related or content provider related.

What is your model Apple TV? Identify your Apple TV model - Apple Support

AirPlay of YouTube videos should be supported.

You may want to inquire with the developer of the second app about AirPlay support.

Not all content is supported for AirPlay. From which content provider or app are you trying to mirror?