Bluing is extremely thin and can't be buffed out the way an automobile paint finish is. Buffing the scratch out will have to be done with buffing compounds that will strip the surrounding finish. Once that has occurred, there is also no effective equivalent to the finish blending that auto body shops do, so, to look perfectly uniform, the whole gun should be repolished and refinished. At that point, for authenticity, you have to keep in mind hot bluing salts come in somewhat different formulations which is why, for example, Colt and S&W bluing doesn't have the same hue; the companies have developed their own proprietary blends of hot bluing salts. So, if you want it to look authentically restored, you should call S&W factory service and ask what they would charge to buff out the scratch and refinish the gun. I don't think that will be inexpensive, though, and you want to keep in mind that for collector value, the original finish, even with the scratch, has more value than a refinishing job does.
For myself, I am more interested in functional shooting guns than collector value in most cases. If the scratch is really small, rather than repolishing the gun, I would do something simple to camouflage it a little and protect it at the same time, like applying Brownells Oxpho-Blue with a toohpick or other fine applicator, being careful not to let it spread around and stain the original finish.
