This is, without a doubt, the coolest way to make a black hole. It's what happens when physics overdoses on poetry and mumbles to itself in an alleyway. It's when light inverts itself.
Most astronomy texts give a quick overview of what goes into the making of a black hole. A star collapses, and so much matter gets pulled down into so small an area that it basically pulls itself into a point. That point has, technically, no volume, but it has infinite density. Venture too close to a black hole and all that mass, squished into infinite density, will pull you in so forcefully that not even getting accelerated to the speed of light will get you out again. And all because there was enough mass in the same place at the same time.
But do we, necessarily, need mass? After all, Einstein showed us that energy and mass are equivalent. Grab two atoms of hydrogen and mash them together hard enough and some of their mass will be converted to energy. So, if energy and mass are equivalent, why can't energy create a black hole?
Technically, it can. Sure, it's hard, because one of the traits of energy (as anyone who has heated up a popcorn kernel knows) is its tendency to refuse to be confined to a small space. But lets say it was. Let's say there was a light so intense, and so concentrated, that it equaled the concentration of mass needed to make a black hole. Some say that this light would, basically, trap itself inside its own event horizon. It would collapse on itself and become a black hole.
We haven't seen it, but it's been given the name kugelblitz - from the German word for ball lightning. So if you see a light suddenly blink off, investigate! You may have found the first physical example. (For safety reasons, we recommend that you not get too close, and use a very long stick to poke it. If the stick gets stretched out and disappears down the darkness, you've found it.)
Via The Physical Review Online Archive