Procession of El Cristo Negro

In the afternoon on October 21, my husband and I heard wonderful jazz-like brass music coming from New York Ave. We looked out our window, and saw a huge procession of people wearing purple, and men who were carrying a pallet with a saint on it. I saw that St Gregory the Great was having the Feast of El Cristo Negro, and I suppose that the procession is part of it. Could someone tell me more about el Cristo Negro? I am really curious about the meaning of the celebration and procession, and would like to learn more about the saint.