For info, see blacknitride.com. For pictures of my examples, see below:
The Black Nitride treatment process does leave a bit of crusty residue, which should really be cleaned from crevices before reassembly, and must be cleaned from the bore if you want good shooting results. I used JB abrasive bore cleaning paste. The Black Nitride treated steel is harder than if it were chromed, so vigorous cleaning is not likely to hurt it.
As for accuracy, this target was shot at 50 yards over sandbags:
I highly recommend the H&M Black Nitride treatment.