How are black holes found?
To add to John Conde's answer. According to the NASA web page "Black Holes", detection of black holes can obviously not be performed by detection any form of electromagnetic radiation coming directly from it (hence, can not be 'seen').
The black hole is inferred by observing the interaction with surrounding matter, from the webpage:
We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby.
This also includes detection of x-ray radiation that radiates from matter accelerating towards the black hole. Although this seems contradictory to my first paragraph - it needs to be noted that this is not directly from the black hole, rather from the interaction with matter accelerating towards it.