Black oxide has two different application methods. Cold and hot. Cold is
for interior show. Hot is approved for automotive exterior use by some
certification orginization and has an offical desigantion for this
use.Never wrote it down,but I guess I will recover it. It is a semi gloss
finish. I'll send a picture of my painted OEM rusty screws. They are not
too bad, but they are indeed painted.Not just the heads. Screws and bolts
are avaliable with painted heads from some bolt sellers.If they dont sell
painted bolts, they all can refer you to a company that does this. They
will drop ship the bolts to the black oxide people.

Mcmaster-carr didnt have metric sheet metal screws or metric carriage
bolts. They offered to search for them for me.I got a call two days later
and they could not find carriage bolts in metric sizes. The sheet metal
screws they offered were mopre expensive than if I got them from over seas
myself.