You can't Nitre blue a revolver; it will ruin it, unless you do not intend on ever firing it again. Any tempering in the frame, cylinder, or barrel, will be taken to spring hardness or less, (570° F. - 650° F.), and that is not hard enough, especially for a thin walled cylinder under high pressure, even in a .38 special smokeless cartridge. Factory parts such as frames, cylinders, and barrels will have a hardness to them similar to gears, (not as soft as springs, but not brittle; around 440° F. - 500° F.), which means a tempering temperature that produces a brown color, and is lower than it it is for springs. Also, any close machine fits could/will warp, and possibly even the barrel's bore along its length, which is why we only Nitre (heat) blue things such as screws, or a part that will not have any force applied to it from firing.
The closest thing that you will get to that color of blue, (having a bluish tinge), would probably be a hot water Belgian blue, or a paint. The standard factory finish, for a S&W revolver, is a high polished black oxide from one of the types of hot bluing. That temperature is no higher than 280° F., which is well below the tempering range, and not enough to cause it to warp.