When a binary system collapses into a black hole, which astronomers call 'coalescence' (a euphemism which makes 'heated debate' a valid description of World War II), it can release a gigantic burst of gamma rays. Gamma rays are the ultimate high energy electromagnetic radiation, and while the burst lasts less than two minutes it can contain more energy than the entire mass of the sun converted into energy by E = m c^2. You'll notice that the mass of the sun and the speed of light, c, are extremely large numbers
Basically, all that garbage Newt was saying about EMP blasts in space causing the electrical grid to be disabled, or all that stuff I said about solar flares washing the Earth in gamma rays...imagine that multiplied by about a factor of...infinity.
On the upside we'll never see it coming. The EM-burst travels the speed of light so the only warning we'd have is dying - which most people will accept is a little too late.
The idea here is that a star 100,000 light years away could have exploded and sent out a wave of gamma radiation at us 100,000 years ago and we won't have a clue until it gets here and wipes us out.
"Here that sound, Mr. Anderson? That's the sound of inevitability." - Agent Smith, The Matrix