Neillo is a type of silver decoration where the background silver is engraved or hollowed out to make the other silver decoration stand out. In the recessed areas a special solution of copper, sulphur and lead oxide is applied that fills and blackens the recessed areas of the silver. This is more than simple oxidation because it develops a thick plaque of oxidation that lasts longer than just black or brown patina on silver (which enhances value when taken off unlike bronze). This is a very common method used on Russian shaskas and kindjals. It can only be used on silver.
An example of Neillo decoration on the hilt of a sword