On alloy and brass parts a fine bead blast, then gently sanding by hand, wet, with 800-1000 autobody paper, will burnish the edges that would see wear, leaving the slightly roughened lower areas that would see corrosion pitting.
On steel,I'll do the same, but try to vary the distance more, to leave a more mottled, varied pitting, and follow with the fine 800 grit sanding on high points. Then cold blue.
A trick with cold blue...each formulation reacts differently with different steel alloys, so you may want to try several. The complication is that "fixing" the cold blue by boiling in distilled water will darken all of them (and more importantly, make them more durable) so determining which cold blue to use on a particular part is not trivial.
You can chemically age steel with vinegar or salt water, and the pitting patterns are more authentic, but the inside/back sides tend to suffer.