You want to rub off the velvet leaving the blue, not rub hard enough to rub off the coat of blue you just put on. The velvet comes right off easy. On this first coat you will have a gray color. By the time you get to second or third coat you will have your blue black color. Now put on second coat of blue with damp cotton ball or swab.
New blue takes off blue, as well as adding blue.
You will see damp metal behind the ball as you wipe, and it will quickly dull and dry. If it beads up, you have too much bluing solution, and you are removing as much old blue as you are putting on new blue. Let this second coat rust about two hours, or till turns brown or red brown, and boil for a good five minutes. Then card off with SOFT wheel or by hand with 0000 steel wool. Your metal should now have a blue black color.