What is the definition of a black hole?
A black hole, is a place where gravity is incredibly strong and nothing can escape, including light (all electromagnetic waves) which are the fastest moving particles we know in the universe, which cannot even escape the gravitational pull.
A black hole is created after a super red giant star collapses inwards, and forms a 'hole in space time'.
We must also know that we have never taken a picture of a black hole, and that EVERY 'picture' we have, is in fact an illustration, and is usually drawn as a 'hole', a 2D object in 3D universe cannot work. In a 2D universe, a 'black hole' would be 2D, a circle. In a 3D universe, a circle is a 3D sphere, which is what observing a black hole would (most likely) look like.
I highly recommend watching Interstellar, as it very good at presenting how relativity and black holes work as well as look. This is a bonus feature from the film, on how they 'made' their black hole look as scientifically correct as possible, and also what they discovered about it.
Hope this helps!