Brownell's Oxpho does a pretty good job on touchups. It is not as durable as a hot blue but doesn't cost you an arm and a leg plus the wait time. I apply Oxpho after I've warmed the metal slightly with a heat gun (or place under a light for a while) and apply with a little 0000 steel wool. You may have to apply several times to get it the darkness you want.
It the wear is more than a small area, like a spot or an edge, I'd leave it. Oxpho is definitely a different color than most hot blues that is noticeable if you cover too much with it and the other cold blues may match better but don't last and often cause rust.