Ruger sells Blackhawk .45Colt/ACP convertibles, which includes an ACP cylinder fitted to the particular revolver. It's not uncommon to hear of owners being able to achieve better inherent accuracy when using the .45 Colt cylinder, though.
You might call and ask if they'll fit a new cylinder to your Blackhawk, which means they'll want the gun. You'd probably be looking at the cost of the part & labor, as well as the not inexpensive cost firearm shipping, though.
As the owner of a couple of convertible Blackhawks - an old model (3 screw) Blackhawk .357/9 & a new model .45Colt/ACP, both in the handy 4 5/8" barrel length - I've seldom used the extra cylinders for either.
It's somewhat anti-climactic to shoot either gun in the lesser caliber for each model. Kind of like when I used to handload and used .44 Spl in my 7 1/2" SBH.
Why not give Ruger a call and ask?