The best fit shape for Sagittarius A*.Source
Black holes have never been directly seen. In fact, they are technically invisible because the gravity near them is so strong that not even light can escape. A new project called the Event Horizon Telescope is putting together a worldwide network of radio telescopes to try to view the event horizon, the area where light is emitted by matter falling into the gravity well, of the black hole. They have focused their efforts on Sagittarius A*, the black hole at the center of the Milky Way Galaxy. This picture here is the best fit for the observations we've made already, indicating that the shape of the event horizons of black holes is actually a crescent. Hopefully we'll see some more of these pictures and we'll be able to draw some better conclusions.
For more information, read this article.