[Milsurplus] replacing black oxide screws /with/ blackened zinc plated screws

[jphutch60bj]

Need to replace a few lost black oxide screws with zinc plated screws.   
I really only need to "blacken" the screw heads.

To accomplish the task I am thinking of Gun Bluing just the heads of the 
zinc plated screws, using a Cold Gun Bluing solution.

Anyone gone down this road before that may have some tips or tricks.

Thanks for you assistance.

Hutch