How bad is the bluing? If it is just worn from normal wear, a reblue job will decrease the value. Forget about it. If the gun is badly rusted or scarred, then the polish job that accompanies a reblue will improve the appearance.
Can you do the preparation yourself? Sure. I always do my own polishing, then turn the gun over to the bluing shop with a "don't touch it" warning. But I have experience in doing that, and usually want to achieve a finish that looks like the original
To do a reblue prep, you have to completely strip the gun. On a Model 19, about the only thing left on the frame would be the barrel with the front sight. Everything else comes off, including the rear sight. You then polish the frame and barrel to the right point, where the finished product will look right. Too much polishing is usually not good. Don't round any corners, and don't "dish" markings, screw holes, or screw slots. You can use a chemical blue remover, but be careful, as it will cause frosting if left on too long. The polish still has to be done, after the chemical has done its work and been washed off.
Polish the crane and sideplate on the frame. You need a special polishing wheel or head to do the flutes on the cylinder.
Do all gunsmiths do a good polish and blue job? Nope. Some can really mess up a nice pistol, which is why I asked how badly you want the gun blued.
Jim