Large black holes do have significant accretion disks that do impact their visibility and can and will spew forth vast amounts of radiation. Anything caught in their event horizon gets pulled in and is never seen again. Anything near it bumps into other things in orbit around it, which lights it up like a candle.

Note that there is also a 'weight class' of black holes that are significantly large that their event horizon would be visible to the naked eye, but which don't have an accretion disk. We currently don't know enough to make scientific estimates on how many of each weight class exists.