Most of the touch up blues that smell (like Birchwood Casey) actually put a thin coating of darkened copper on the surface of the metal. Kind of like painting a finish on the metal.
Yes, this type does smell and it does wear. This type will often even wipe off on your hands. Depending on the metal, some take it better than others. Either way I don't recommend or use this type.
Van's Instant Gun Blue DOES NOT smell.
Vans is the best touch up blue I have used. It will blend in your holster wear if it is not too horrible and it will wear fairly well. Of course repeated placing and removing gun in the holster will only cause the wear marks to come back over time. Vans will penetrate the metal as opposed to paint on a finish. It will not penetrate as deep as conventional hot bluing. So the finish on the touch up areas will be thinner. But for touch up's it works great. The use of this blue will certainly enhance the appearance of your gun that has over time begun to show finish wear. It will not provide the durability and deepness of color that factory hot bluing will.
Depending your you ability/technique to apply, it can make an old gun appeared better cared for that it really was. Meaning it can be used to enhance (deceive) the appearance of the original finish. The blending on some gun metals is so good that under normal (poor) gunshow or gun shop type lighting, you will have a hard time telling it was touched up. But under extreme light using magnification, you can identify if it has been touched up if you know what to look for. Otherwise it just appears that the finish is thin in some areas.
There is another I've heard good things about but never used called Formula 44/40.
I do not condone the use of touch up blue in an attempt to enhance the appearance of a gun for higher resale or profit. That, in my opinion is unethical.
I have used 44/40 and it's really good stuff. It will give you a true bluing color. Like any other cold blue it wears, especailly around edges wear most wearing occurs.
The biggest problem will be in trying to match the original clolor. You just have to keep re-applying to get the darker finishes.
I prefer to see the honest wear than to have a mixed, uneven finish that will wear off with any reasonable amount of usage.