Sounds like bad pre-chome plate prep'ing .. inadequate pickling maybe and/or nickel stage. Doubt much you can do therapeutically.it's underneath the plating in the form of black specks.
What I mean is ... try it! It is totally safe if you want to try and phosphate some light rust .... tho probably slower than using a proprietory formulation.Did you mean, "don't put that to the test?" Or is it safe to try it?