"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 

"A negative to the black oxide finish is the fact that the coating is not a corrosion- resistant finish. It is not suitable in any situation for outdoor use, a fact that negates its selection for stainless steel in the first place. "

"Mcmaster-Carr , MSC, WW Grainger,ect dont sell metric sheet '
metal screws. "

That's beacuse they do not exist. With the exception of the head dimensions, a Type AB sheet metal screw in Europe is the same (thread) as the type AB screw bought in the US.  A metric desigantion is used, such as 4.2mm (#8) or 4.8mm (#10), but it's the same thread.

"OEM screws are plain steel and  just painted black"  

I never seen production screws for any automotive applicaion that are painted. 

John