Yes, one thousand black holes have been seen. In reality, beams of light have been captured and these are assumed to mark black holes – see www.dailygalaxy.com/my_weblog/2013/04/image-of-the-day-1000-monster-blac…
It claims material falling into the black hole generates the flashes of intense light that are detected by optical telescopes. Black holes, it is theorised, are galactic nuclei – and every galaxy has a black hole for a heart.