What exactly is blackhole? A region of space where gravitational field is so intense that no matter can escape from it. Not even light can escape from it. The place at which the entire mass is concentrated is called singularity. In upper image the black area boundary is called event horizon. Outside event horizon of blackhole if any body comes it can easily escape gravitational pull. If body enters event horizon then it wont be able to escape from gravitational pull because escape velocity becomes equal to gravitaional pull.
: Light does not comeback after entering event horizon. Hence no light emitted from it.
:Can’t be observed directly by telescope situated on ground. (But Presence of blackhole can be infered through the interaction with bodies. Like star revolving around invisible body. A perticular star revolving around invisible body tears apart because of pull. This lead to emission of X-rays which can be detected. These all events can denote presence of blackhole)
For further understanding watch video: