Trust me on this one, if you just have some scratches or real small spots then what you can buy over the counter is just fine.  If you want a job that looks good and is going to last take it to a pro.  If you try it yourself you will never be happy with it. 

I've used Birchwood Casey cold bluing products for touch up with good effect. However the cold blued finish is not going to hold up as well as a finished applied through hot bluing. I've not done any hot bluing but there are gun smiths in our area that offer the service, maybe there are some in your area.

I once sent a revolver back to S&W to have it re-blued through the factory and while it took awhile the resulting finish was outstanding. You probably could do the hot bluing yourself if you have the patience, time to research, and an open ventilated space away from your home.

I don't bother with touch-ups for minor wear or dings.  Any slight wear marks, or scartches just add character to your rifle.  If that entire blueing has turned to plum color or brownish, or if a lot of surface wear has occured, then you can just have the whole rifle stripped and re-blued.  Hot blueing is the best way to go for any blueing.  Might want to consider a phosphate parkerizing for a more protective finish.  It's dull, matte dark charcoal grey in color (almost flat black) and what some say is ugly, but it's a very effective and protective finish.

Either way I'd suggest staying away from the cold blueing products on the market.  All they do is leave a marbled affect on your rifle and don't blend well with the existing blueing, plus they aren't very durable or long lasting.  You'd be better off just having a gunsmith remove the existing finish and then hot-blue your rifle. 

You know what modern gun blueing and black-oxide finishes are, don't you?  They are both oxidation finished, simply put they are a form a controlled rusting.