There are any number of ways to age a screw.
If they are new replacement screws you can take a brass brush and remove part of the blue across the top of the screw head. Throw it in you pocket with your change and walk it around for a while and it will take a little more off, or even out what youve done with the brush. Or burnish it with some leather. Take it slow and compare to your other screws.

Depending on whats desired, Ive used plumb blue–dip in and immediately wipe down screw till you get desired color. Ive also immersed the screws in hot sauce, ketchup, mustard, or vinegar for a few hours or over night, then place them in ice trays and sat them in the sun for a few days to develop a little rust on the surface–then wipe down and clean. The acids will etch the metal surface. There are any number of ways to do it, just depends on the finish color or blue coverage you want. Im sure there are others out there that may have better or additional ways of doing it.