Best solution is delete the current version, reinstall the app, restart, run the app, let it update, restart again, and it should work.
Via DinnerBone on twitter.
I have the same issue. On my Retina display the stream works, but it is very gittery, so I switched over to my Hackintosh that has no problem running any mac app at all, and thought the it would do better with a better GPU streaming instead, and it gives me a black screen. Cleared caches, even changed which GPU MC is launched with and its all Black screen.
Twitch reports a "Excellent" stream quality, audio works fine, just no video. Hope it gets sorted soon.
Best solution is delete the current version, reinstall the app, restart, run the app, let it update, restart again, and it should work.
Via DinnerBone on twitter.