Locally, there is a large amount of gas and dust attracted by the black hole's gravity. As this material approaches the black hole, it swirls in a vortex, like water swirling down a drain. As the material is packed together and falls into the hole ( passing inside the Swarzchild radius when we can no longer see what happens to the material) it emits large amounts of radiation. This, plus re-emitted light as this radiation heats up surrounding gas and dust, is visible as a bright shell or disk around the black hole.