Most of the aftermarket nipples are either stainless steel which does not take bluing or made of a special very hard bronze called Ampco which also does not take bluing.
There are a few nipples that are made from regular hardened steel which have a black appearance.
These nipples usually wear out faster than the previously mentioned stainless or Ampco nipples.
Check out the nipples at Track of the Wolf
https://www.trackofthewolf.com/list/Item.aspx/807/1
Be aware that nipples not only come with different materials and thread sizes but they come in several different sizes.
The largest is a Musket nipple, made for use with Musket caps like those used in the Civil War.
The smaller rifle nipples use the same #10 or #11 caps as the pistol nipples but they are longer from their shoulder to the top of the cone.
These will screw into a Cap & Ball cylinder but they are too long to work correctly.
Then there is the pistol nipples which are shorter than the rifle nipples.
These are made for the Cap & Ball pistols. (Single shot muzzleloading pistols use the longer rifle size nipples).
Then, there are a few special size nipples made for pistols made by some small companies and those made for underhammer rifles.
Unless you have one of these guns, don't buy these.