I suspect that your NM .357 Mag Ruger Blackhawk has a 6 1/2" barrel, and not a 6" one. At least, I canot find a 6" barrel .357 Mag online. At any rate, why are you thinking about changing the rear sight? It is easily adjustable, and very easy to see thru as well. I have had a OM Ruger ,357 Mag Blackhawk (6 1/2" barrel) for over 50 years, and the sights have never been any issue. I use to shoot at swamp rat mounds at 100-150 yards, and could tell if I hit it or not by the nearby splashes. Other than when I made some reloads too long, the gun has been flawless. I now also own a .45 Colt Ruger Blackhawk convertible (shoots .45 ACP rounds with a cylinder swap) as well.
Both guns have the same style sites, and both are very accurate on the range, where I shoot up to only 50 yards, because beyond that, it is near impossible to see the target! My targets are 1 sq. ft. each. I normally shoot at a distance of 25 yards, and have adjusted the sites to be dead on for that distance. If I shoot at the 50 yards again, then I have to aim just a bit higher.
If you really want to shoot long distances normally, then put a scope on it. I have a friend who had put a scope on the Ruger .357 Mag, but I never shot it. Of course, the scope adds weight!