I have never heard about the existence of black hole clusters. There are certainly super-massive black holes, but black holes are generally found at the centers of galaxies and certain star clusters. Indeed, because of their intense ability to warp space/time around them, stars and other matter accumulate and swirl around black holes. And while there are clusters of galaxies, there are no clusters of black holes alone... at least none have been found as yet. (And one would think that as soon as black holes got close enough to cluster, they would swallow each other up.) By the way, it is believed that the center of our galaxy, the Milky Way, contains a black hole.
And just as a matter of correction, sanjiwan, there are no black holes known to exist in our solar system. If any did exist, we probably wouldn't.