From the virtual pages of The Mac Observer:
To recover from Black Dot and get the Messages app working again, here’s what to do:
- Swipe up from the bottom of your iPhone display to see the app switcher, then swipe up on the Messages app to quit it. If you have an iPhone X stop your swipe gesture about a third of the way up the screen.
- 3D Touch the Messages app icon and choose New Message
- Tap Cancel
- Swipe right to left on the Black Dot message to delete it
If you don’t have a 3D Touch-capable iPhone, you can use Siri to send a long message to the conversation. Now go back into the Messages app and delete the conversation.