Cheap anodizing.

[rz5mark]

The GSX-R1100 oil lines have a black anodized look when new 35 years ago. Mine were pretty weathered and tired looking. My Dad used to do gunsmithing and put me onto "cold bluing" a bunch of years ago. It's not anodizing but I think it looks better than painting satin black. Youtube has lots of info on it of course.

[hardrockminer]

Several parts on Z1's were anodized, most noticeably the brake reservoir.  They had a really nice black/purple sheen to them.  Most of them fade badly in the sun over the decades, and restorers are left with a decision to either paint it or leave it alone.  I know one guy in Oz that has successfully re-anodized but it's difficult and complicated.  I have a couple of reservoirs here at home and may try your method.

[rz5mark]

You have nothing to lose. Lots of "cold Bluing" tutorials on youtube.