I use a blackener made for the type of material I am wanting to darken. For instance, Brass Black from Birchwood Casey will work on brass, nickle silver, but I am wanting to stain Stainless Steel, I use a Stainless Blackener from Caswell. If it's steel such as the spine of a blade, I might use baking laquer.
There are lots of tricks to achieve the same thing, so let's see what these guys say too.
Pins are sometimes threaded, but some are piened. I rarely ever just glue a pin.
Lin