Rugers are not known for their accuracy on a stock Blackhawk. Mine with factory rounds held across sand bags could match yours but with reloads and proper sized bullets (.454) for my gun, I have been able to cut that down to several inches at 25 yards.
Mine also has the second cylinder for 45 ACP. I have a hard time holding 10 inches at 25 with that cylinder.
Several things you can do to see if it’s the gun or you. Have or you check the chamber throats in the cylinder. There are a lot of stories about the throats on Ruger Blackhawks being inconstant and can cause accuracy problems. There are also tools that you can use to make all 6 the same diameter. Mine was built in the early 80’s and the throats were cut so that the diameter are all the same size.
The second is to have someone else shoot your gun to make sure you’re not the problem. I had a light weight Kimber that I couldn’t hit the broad side of a barn and found it was me. The short 1911’s don’t fit my hand very well.
This is my evil devil gun, notice the serial number?
"The difference between stupidity and genius is that genius has its limits" Albert Einstine
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