[dgludwig]

Have you had any other competent shooter try shooting it from a rest? It's not very likely but sometimes some guns for some reason just don't work for some people. That about "sums" up my long shot suggestion but I'd want to rule out any possibility before giving up on the gun. And I'd certainly make sure the sights aren't moving around.
Based on my experience, it seems very odd indeed for a Ruger Blackhawk to "shoot all over the place" from a rest @ 15 yards. The good side is that Ruger has a well-deserved reputation for treating their customers right. Keep us informed as to how this gets resolved.