Well like I said before if you want black it will have to be Black Zinc or a Black Anodize.
If you do all stainless steel bolts with black oxide the finish will eventually go away it's not a long term type plating.
If you do black oxide on steel parts you will eventuall have rust.
So the only other choice is the painting the heads of the screws but again it will chip off.
It's up to you and you may find a specialty sop that sell that type of finish but it will take some looking in to online.
Good Luck.
If you do all stainless steel bolts with black oxide the finish will eventually go away it's not a long term type plating.
If you do black oxide on steel parts you will eventuall have rust.
So the only other choice is the painting the heads of the screws but again it will chip off.
It's up to you and you may find a specialty sop that sell that type of finish but it will take some looking in to online.
Good Luck.