This is what I do except I use a hobby knife. If the wheel is not white, I will paint it a flat white.
The trick that I have found is to use the blade at a very shallow angle to the wheel - like about 60 degrees. The more vertical the blade is to the wheel, the more likely it will dig in or gouge the paint. The last thing is to wait until the black paint has thoroughly dried. If the paint is not thoroughly dry, it will likely chip in places and you no longer have a clean line. As another board member writes from time to time - "don't ask me how I know".