Use black steel fittings for steam generation, oil, and non-potable water. They are useful when you don't want to use galvanised steel, such as for hydraulics, power generation, refineries, and other general engineering projects. The black finish is created in the manufacturing process after casting. There's a scaly, dark-coloured iron oxide coating on the surface that is useful in the above applications but it makes these fittings off limits for transporting potable water. Read more about black steek fittings.