Withe gun unloaded (naturally) and fieldstripped, look at the frame casting near the firring pin and recoil shield.
1. Ruger frames are investiment cast and can be very rough and porous in this area. Do you see any burrs or sharp edges where the cartridge rims can catch?
2. Examine the cylinder ratchet and the hand or pawl in the frame that rotates it. Any burrs or sharp edges there? Any unusual wear?
3. Look at the tip of the transfer bar (in front of the hammer). Does the transfer bar catch slightly on the firing pin as it rises when the hammer is cocked?
Those are the most common places for a New Model Blackhawk to bind when the action is cocked. Deburring and smoothing the action parts is a fairly straight-forward procedure. But if you do it or have a gunsmith do it, the warranty will be voided. I work on my own guns, but it's your choice.