What does Plain Black Uncoated mean?

Product where the term "Plain Black Uncoated" is in the title or description means it has no actual protective coating other than maybe a light oil residue. Stock is sold in the bare steel unfinished state (basically straight off the machine it was made on). It has no corrosion resistance and will rust over time.

It is often confused with Black Zinc, which goes thru a zinc plating process with a black coating that is applied over the "unfinished" product for protection and appearance.

The term “Plain Black” is an old industry term plain finish, simply meaning no coating has been applied to the product after it has been manufactured. 

Depending on the material, grade and heat treatment process it has been through the actual colour can vary from a shiny oily finish to a dull black or grey.