""Black oxide is the premier finish for metals exposed to the elements.It is
top of the line for corrosion resistance and durability"

Beg to differ, but I've never known black oxide as being chosen for corrison
protection and I been working with fasteners since 1978.   As proof, I quote
Tom Doppke (Former GM Fastener Engineer) below "

Agreed. Not some great magical coating. My understanding is that the black
oxide is used for three reasons: One, it's easy as a conversion coating that
can be performed post heat treatment.
Two, it absorbs oil since it is porous so they tumble them with oil and
they're protected enough to make it into use.
Three, it's cheap.

Black oxide looks nice out of the box, but it usually ends there unless they
are internal engine fasteners. If you don't mar the head badly they can
offer better than none protection, but I certainly wouldn't use them where
corrosion resistance is a factor.

Jim Akiba