I've blackened my original steel carriage bolts for my centipede trackball using that gun blue stuff (worked pretty well - wish it was a bit darker, but certainly better than before).
Now the question is, how can I blacken aluminum screw posts? The gun bluing stuff says it only works on ferrous metals. So is there anything I can do to blacken aluminum in a fairly permanent manner? I can live with the silver look if not, but my overlay lends itself to black hardware more than silver.