I will go against the grain here. I certainly wouldn't touch it with emery paper, or any other abrasive. However, touch-up bluing shouldn't hurt anything. I've done it dozens of times, with good results. Just remember that the spot will still be visible...at least to you. But, it doesn't have to stand out like a sore thumb. Just keep in mind that all you are trying to do is darken the steel. Dark steel won't show up nearly as much as the silver steel you are seeing there.
Clean that area thoroughly with acetone. Any oil (even fingerprint) will cause problems. Heat the area with a hair-dryer. Wet a Q-tip with the cold-bluing solution and rub it into the area. Try to keep it away from the finished areas (that's why I suggest a Q-tip instead of a cloth). It won't really hurt the existing bluing, but it will probably dull it just a little. You should notice almost immediately that the bare steel will turn black. Keep rubbing solution on it, keeping it wet. After three or four applications, wipe the solution off with a cloth dampened with water. Let it dry and gently polish the area with 0000 steel wool. You should see a marked improvement in the appearance. If you want it darker, repeat the process. Once you are happy with it resist the temptation to apply oil just yet. Let it sit at least overnight. Next day, rub some good-quality oil onto the gun and the spot should be much less noticeable.
