A Black Hole is a region of space-time exhibiting such strong gravity that nothing — not even light can escape it. It contains a huge mass in an infinitesimal space, where gravity and density become infinite, space-time curves infinitely, and laws of physics no longer hold.
Black Hole is not visible to the human eye. Specially designed instruments like a Telescope can detect a Black Hole. Also, they are identified by virtue of strong gravitational effect that they have on surrounding bodies or light rays.