As CraigC pointed out, the original .357 Magnum Blackhawk was made on what is now called a mid-sized frame. It also did not have a transfer bar safety to prevent an accidental discharge if the revolver was dropped, and hit on the hammer while there was a loaded chamber in front of it.
In 1973 Ruger revised his line of single action revolvers, and included a transfer bar safety to prevent the accidental discharges referred to above. At the same time he discontinued the mid-sized frame, and thereafter made the .357 Magnum version using the same frame and cylinder size that had been developed for the .44 Magnum.
So what you have is a .357 Magnum built on the larger .44 Magnum platform, and you also have the transfer bar safety so you can carry it with the cylinder fully loaded with 6 cartridges. Therefore go forward and not worry about any .357 Magnum ammunition made by any company, or any listed handloaded data, although I suggest you avoid the "Ruger or Thompson Center Only" loads unless you are a very experienced reloader who understands how to evaluate signs of excessive pressure.