Blank/Black Screen on AppleTV when mirrored from iMac Using macOS Sierra 10.12.5, iMac 27-inch Mid 2011, AppleTV Model A1469 7.2.2 (8015)
Connected via WiFi

I have an Apple TV that used to work, however, now whenever I mirror or extend my display from my iMac it only shows a blank/black screen with audio. 

I found this post, Apple TV Black screen (OS X Yosemite ) however, the solutions there did not work for me, and I am not sure how it worked for the OP.

I also found How to Fix an Apple TV with a Blank Screen and reset my Apple TV to factory settings, but still the same problem persists.

The audio works fine and I can switch between internal speakers and the AppleTV but there's no video on the AppleTV.

Any ideas at all are greatly appreciated...
I had a similar issue while using an Apple TV to connect a TV to my MacBook Pro (via LAN) and playing around with the different display modes as it suddenly stopped working.

Unfortunately, I don't remember any details any more but (after several failed attempts) I solved it by explicitly selecting Off in System Preferences/Display/Monitor/Dropdown Box Airport-Monitor (maybe detaching and reattaching the ethernet cable and/or turning off/on Apple TV were also involved) and reselecting Apple TV again.

This might be important, as deliberately selecting might have reset and/or stored parameters, which may explain why some people experience negative results.

Sorry, if this was too less of information.