I have a 7 1/2 inch Blackhawk in .45 Colt I like a lot, and I find I seem to shoot the longer barrels better. I doubt they're really more accurate inherently, I just seem to shoot them better.
100 FPS with a 255 grain bullet should do the job from about any angle and reasonable distance. I have found shooting stiff loadings in a Blackhawk that my willingness to withstand recoil and muzzle flash is the limiting factor more than the strength of the gun.