If it has that dark patina on it you must get it off with the Scotchbrite pad first or it will show through the blue. Proper metal preparation is the largest part of it.
Get a $20-$40 beater rifle or shotgun and play with the take down to metal and bluing and see how easy it is. It gets rid of the nerves and builds confidence.
Im sure if you mess up you can strip it back down and try again. I paid 50 for that 311 so I could tear it down and build it back up that way when the heirlooms go down I'll be able to change out parts without destroying them. Once my project list shrinks I will have to look into doing it myself.