The answer is that we can't see a black hole, which is I would guess what you were leading up to. There are only two ways we can detect a black hole using light (of various wavelengths):
if it occludes something behind it
if it's surrounded by an accretion disk
Point (1) is actually quite complicated because a black hole doesn't simply mask whatever is behind it, but instead it acts as a gravitational lens. This can cause various optical effects such as microlensing. I mention microlensing because it has been used a way to search for small compact objects. We haven't found any black holes with this technique, but we have found several extrasolar planets.
Point (2) is how we find candidate black holes such as Cygnus X-1. When the black hole is part of a binary system it can accrete matter from its companion. That matter is heated by tidal forces and radiates X-rays. Likewise we believe that quasars are supermassive black holes accreting matter from the galaxy around them. The intense radiation of a quasar comes from tidal heating of this matter.
Response to comment:
In our coordinate system the matter faling into the black hole never crosses the event horizon. As the matter approaches the event horizon it appears to move more and more slowly, and the light from it is increasing red shifted. However the matter will never fade completely to black. That's why we can still see it.
When I say in our coordinate system this sounds like a technical point, but in our coordinate system means as timed by our clocks and as measured by our rulers, so it's a real effect. As we measure time it would take an infinite time for anything to reach the event horizon let alone cross it.