jamisobe, Unfortunately all of the screws and washers supplied with the pocket flares came under attack from the effects of corrosion... maybe it's where I live in SoCal. The supplied screws I removed, to remove corrosion and repaint so they looked better than the washers. The corrosion was starting to bleed through the paint on the screws though.
Without looking at my receipts I think I spent around 40-60 dollars for stainless steel screws, washers, rivnuts, (bought online) and rivnut gun bought at Harbor Freight.
I'm sure the black oxide bolts could be used with the rivnuts, but my bolts were corroding as well. For long term use I bit the bullet and went with stainless steel. The added benefit of easy on easy off with the use of rivnuts, has brought these pocket flares to whole new level of functionality. Damage prone or tight trail... well now you can easily remove your flares.

You want to clean that 100 miles of trail dust or mud out from under your flares... you get the picture.
My advice for those thinking of going this route is to practice setting a couple rivnuts on a spare piece of metal to get the feel for setting the tool, use high quality rivnuts, and to take your time during installation. Hope this helps.