I prefer simple and inexpensive solutions. Most of the time, I choose the simpler of 2 options, but that's just me.
I use lemon juice from the grocery store mixed with water. I don't measure but it's about 2:1. It can be had dirt cheap and it's worked perfectly for me. Obviously, the active ingredient is citric acid. I use red and gray scratch pads to remove the black and check the progress. Then I rinse with water and immediately apply a coat of WD-40.
My needs are basic. I want to remove surface rust such that the tool can be put back in service. My tools are all users so I don't care about preserving etchings and such.
-- Dan Rode
"We are what we repeatedly do. Excellence, then, is not an act, but a habit." - Aristotle