If you are trying to watch Twitch stream but get only black blank screen instead on your mobile device, go to Options by tapping on the gear icon, and here try to switch Quality from Source to Auto and back, until you get the stream.
The quality selection option on the stream itself is only visible when there is enough viewers, or the streamer has partnered with Twitch. Twitch partners has the quality selector always active. On older Android devices where official Twitch app doesn’t work well, better 3rd party clients are available like Vortex. If you are experiencing blank screen when watching streams on your computer, just refresh the webpage by clicking the refresh or reload icon near the address bar, or just press F5 keyboard button.