Why can't we see black holes? 
*

*Is it all because they are so black? 

*But even our tech is not able to see black holes. We can only tell where black holes are, when they are eating stars. Or it's because they consume light?
 A: Today, seeing an object is pretty much done by capturing the electromagnetic waves emitted or reflected by that thing and forming an image of that object. Black holes do not reflect or emit electromagnetic waves (except Hawking radiation which may be too small amount for seeing purpose). The reason for not reflecting and not emitting is that the enormous gravity of the black hole does not let anything including light, escape from it.
However if scientists succeed in detecting the gravitational waves, then it will be possible to see some black holes in form of gravitational waves. Until then, black holes can only be detected due to their influence on light and nearby stars.
A: By definition a black hole is an object so massive that its gravity prevents anything from escaping it including light, once inside its event horizon. Its effects can be observed, however. It would appear as a black sphere against background stars and the like.
A: The event horizon of a black hole marks the limit of a gravitational field so strong that any known object in this universe, including a photon of light, would need to attain escape velocity greater than the speed of light in order to overcome the pull of the gravitational field.  As the speed of light marks the upper speed limit in this universe according to General Relativity, light can not achieve the required escape velocity.
Outside the event horizon, the effects of a black hole are visible.  Light from those effects eventually will reach an observer.  But inside the event horizon, photons proceed toward a future that is invisible to an outside observer.
A: Every object has an escape velocity, the velocity that if given to an object, said object will never fall back. On Earth that is 11.19 meters per second. For a black hole that velocity is greater than the speed of light or 299,792,458 meters per second. If the escape velocity is greater than that, then light does not have sufficient velocity to escape from said object.
A: Yes, you are right...We see object because when light hits the object, it reflect and reaches our eye through space, of which image is made at Retina..
Black Hole's gravitational field so strong that not even a light can escape from it...Though we cannot see a black hole itself, but we may see the hole's effects on nearby matter.We can detect it by help of X-Ray telescope (like.. Chandra X-Ray Observatory)
So, if gas from a nearby star were sucked towards the black hole, the intense gravitational energy would heat the gas to millions of degrees. The resulting X-ray emissions could point to the presence of the black hole.
Black Hole emits huge amount of X-Ray's, but our atmosphere absorbs mainly of it, so it is very difficult to detect X-Ray down here, so we have space-based telescopes to detect X-Ray sources from far end... 

Sources :  BlackHoleHowSee,  Chandra_X-ray_Observatory,  X-ray_astronomy_detector

A: Apparently black holes are "visible". Of course not by means of "photons" (as already stated in previous replies) but of gravitational waves. 
By means of numerical simulations the authors were able to constrain very accurately the dynamics that led to the formation of the black hole as well as the masses of the stars involved.
Ligo and Virgo scientific collaborations, Detection of Gravitational Waves
