In new model Ruger revolvers, it is safe to carry 6. In the old models, or anything that does not have a transfer bar safety, DEFINITELY only carry 5.
However, for the purposes of gun reliability, in both New and Old Model Blackhawks, you SHOULD carry and fire the gun with 6 rounds (1 can be an empty case) in it.
The reason you need a cylinder full of cases is because unless the loading gate has a cartridge case in front of it, the gate is unsupported and during recoil can move forward and disable the gun. This is not a problem with Rugers that have custom cylinders with recessed case heads because there is less gap between the loading gate and breach end of the cylinder. When the gun recoils and tries to push the gate forward, it does not have to travel as far forward before it hits the rear of the cylinder.