Magnum cum loude, Some people load lead bullets in their 9mm ammo ... I don't. All my 9mm ammo gets jacketed bullets and there's a good reason. When I grab a box of ammo, I want it to work equally well in any of my guns chambered for that cartridge and I don't "tune" handgun ammo for a specific gun. When the German Luger was first designed, it had a 1:10 twist rate specifically for jacketed bullets so that became the standard twist rate for nearly all other brands of 9mm pistols. Lead bullets don't play well in a fast twist bore ... almost always resulting in excessive bore fouling. You can get by with lead bullets in a Blackhawk because they have a slow twist rate of 1:18.75 ... almost half that of a 9mm pistol. So ... if you use lead bullets in a 9mm Blackhawk, you may be able to get accuracy almost as good as a 38 Special. The bore in a Blackhawk is rated at .357" +or- .0005", however the cylinder chambers are tight enough where a .356" lead bullet is the max diameter that will fit without having to drive cartridges in with a hammer. As such, 9mm bullets don't seal as tight in the bore, thus a loss of pressure and velocity, plus a reduction in accuracy. If you happen to get a Blackhawk with a bore at the tighter end of the spectrum, it may shoot 9mm ammo quite well, however I have tested several dozen and never found this to be true.
Obturation is a function of chamber pressure versus bullet hardness (rated in Brinell Hardness Number or BHN). The formula is BHN = chamber pressure (in psi) divided by 1400 ... or ... Chamber pressure = BHN times 1400. Typical 9mm jacketed bullets are rated at BHN 32 so to get them to obturate, pressure must be (32 x 1400=44,800 psi) about 45k psi. Hard cast lead bullets are typically rated at BHN 20, which is just about right for a 9mm cartridge with a chamber pressure of 28k psi.