If you know the cross you can make black rocks too, so by the sound of it someone either bought day-old black rock chicks and sold them to you or they bred them themselfs. Either way. a good laying hybrid. Usually have a nice temperament.
They are all mutts, just some look more like the rir parent.
What one of the hatcheries up here sells as black rocks are black sexlink hybrid layers(rir over barred hen), by the sounds of it thats what your hen is.