How do people keep the black color from the forge on the spine of there blades?

[Fox]

I always like that look, but the black color from the forge always pops of when I quench it, do you have to clay the spine or something?

[White Falcon]

I don't know. I am doing a blank that was black when I got it. I am using black scales. I did change the profile to my liking.

[White Falcon]

Sorry for the double pic.

[Gimlis Ghost]

Any time I've oil tempered a blade the steel turned black. In late Medieval times they stopped polishing surface hardened plate armor so untouched armor from that period was black rather than polished. Most of that armor was later polished bright when restored for exhibition but period paintings reveal it was jet black.
I suppose the black coloration was a selling point, proving it was surface hardened and not old enough to have the surface finish worn away. It also, when oiled frequently, acted like gun bluing to reduce rusting.

I suppose the color after tempering depends on the type and quality of the oil. I always used burnt motor oil, sometimes from a lawn mower. The contaminants may be why the blades turned black. I generally polished them bright after finishing the blade.

Case hardening of cast steel or iron gun frames can be done using bone meal and leather craps to produce a multi colored finish that is quite striking.
Kasenit usually produces a medium grey finish.

[Mr. Woolery]

I don't mean to necro an old thread, but this was something I had to figure out as well. 

When hardening, my surface would pop off the light scale and I'd have a shinier surface.  But if I had oil on the blade when I went to temper, it will normally polymerize on the rough surface (actually all over, but I finish grind the bevels and it is gone).  That polymerized oil is dark and looks very hand-forged. 

I discovered this because I didn't want that surface.  I wanted clean steel.  If I scrub off all the oil with Dawn dish detergent, I end up with a surface I can hit lightly with a wire brush and get shiny forged surface. 

Your quench and your experience may vary significantly.

-Patrick