We cannot see black holes in the sky because they are completely dark. They do not shine like a star, nor do they even emit radiation like a planet. They do consume light and matter, which makes them ever so interesting to astronomers and the public alike.
For example, if we were to shine a flashlight at a black hole the light would not reflect back at us or get re-emitted at some other wavelength. It would just disappear. As such, it is a challenge to find them.
We do think that massive stars are capable of becoming black holes at the end of their lives, and that these stellar mass black holes can grow by gathering up the material that falls onto them. There is the common misconception that black holes "vacuum" up any material that gets near it. This is actually not the case. A person can lead a relatively normal life on a planet in orbit about a black hole, as long as o has soneme assurances that this orbit is stable! It is only when material falls inside the point of not return, or "event horizon," that it becomes lost to us.
It is interesting whenever we have the chance to witness a star like the Sun as it passes through the event horizon. If we can just catch a glimpse of such a star just as it is falling across that treacherous point of no return (and not well before or well after), then we can learn more about the attributes of these dark and silent monsters in space.
Yesterday astronomers think they have just seen such an event. We call it a "tidal disruption." What we think is happening is that the bulk of this giant ball of hydrogen in space which we call a star is consumed by the black hole. At the same time, some of this material is accelerated to high speeds and ricochets off of the accretion disk just in time to miss out on being eaten and to make it to our telescopes here on Earth. We see giant red flashes of light from Earth and then darkness as that little bit of light is spared while the rest of the star quietly disappears from our night sky forever.