From what I could see in the picture, you will first have to clean the area up with 600 grit or higher sand paper ( i would use 1000grit). You will have to remove the firing pin from the slide, and use a flat needle file with sand paper wrapped around it. Once the area is clean of all rust and associated minute pits, then proceed for bluing. Else, rusting would continue internally and just resurface at some point in time.