The retired gunsmith who taught me the pencil trick, grabbed a yellow pencil laying handy. As I recall, it was a #2... like most school pencils were. As said, he warned me about the "ink erasers" with grit in them. Same as sand paper. For this a total NO-NO. At one time I tried a "art" pencil that was a #1, maybe with separate eraser. Didn't do a thing. I believe the higher numbers are harder... #3, #4... I have "cut" rust, light rust, with just a green eraser, the kid in school kind... Never like "art gum" for anything, too crummy.
Point, there are a million versions of blue, It varies with the exact quality of chemicals that went into the tank, the temperatures, the steel being blued, THE preparation... many a blue job has a fingerprint in it... I was always scared of that English blue that was supposed to be so soft, BUT real "blue" color, that you could rub it off with your thumb... If you have a water spot that has turned to a pile of soft, brown rust, that comes off easily. Otherwise, it is a gamble. Sometimes it does not work.
"Blue remover," at least the one I used long ago, was straight nitric acid and said so on the label. The blue came off like mud under the garden hose. I learned about Gunk the hard way too, but it was a concealed area. And some bore cleaners are hard on blue. You can pay the pros for a system they know how to make work, or you can "mess around." I like to mess around. enjoy...