I'm not sure if I've got your story straight here: You BOUGHT the Blackhawk with a cartridge stuck in the cylinder? OR You bought it knowing there was some damage to one of the chambers??
Cases can and will stick in a chamber if there is a gouge or scratch in the inside wall of the chamber. Generally, the brass will flow into the void upon firing and the become stuck. It seems to me that polishing it enough to remove the scratch will widen the chamber and you don't want to do that. However, if there is a sharp edge to the scratch, then perhaps smoothing the scratch a little will make cartridge removal easier.
As for your flattened primer, generally, that's just a sign of hot loads with too much pressure. Your mentioning of the primer is where I'm lost and get the idea that you bought the revolver with a cartridge stuck in it.
Please further explain your situation. It may get you better feedback and assistance with your problem.