This may be a DRM issue.
Hollywood is big into shoving HDCP DRM into everything, so you can't make copies of video, unless they allow it (e.g., temporarily, on your cable company's DVR). If this MacWorld article is correct, it appears that programs other than iTunes can't verify that your monitor is crippled with HDCP. When they can't, they intentionally refuse to play 'protected' video, and you get the black screen.
https://www.macworld.com/article/228703/oops-you-cant-play-most-protected-digital-video-on-an-external-mac-monitor.html
If the article is correct, Apple could provide a way for programs other than iTunes to do this check. Or Hollywood and the streaming providers could remove the DRM … like the record companies eventually did with AACs and MP3s sold through the iTunes Store and Amazon MP3 Store. But I wouldn't hold your breath waiting for the latter to happen.