eburacum has the correct answer according to mainstream theory, except there may be no (or few) black holes with less than two solar mass. A thousand tons per second of mass converted to energy would be spectacular at a distance of one million kilometers, but would probably go unnoticed at a billion kilometers or more. Some black holes (of all sizes) have dense accreation disks. The collision of accretion disks could scatter lots of debris and gamma rays, nutrinos and lesser photons.<br /> If there is negligible accretion disk, perhaps there would be nothing to see when two black holes collided IMHO. Neil