I just stucka couple of stainless dome head allen bolts in my old bike, and my new bike come to think of it If you go on flea bay etc, you can actually buy black anodised bolts, as I did later, but have no pics. Not sure about blanking plates though ?? Good luck
I use black nylon phillips pan head screws. Work great, no corrosion, look good. Do your passenger brackets have the pannier brackets on them as well? I'd be interested in buying them if they do.
Thanks for the suggestion Yes they have the brackets but will hang on to them as I do have the panniers which I use occasionally Cheers