When referring to "black steel," it's important to clarify the specific context or material being referred to. There are a few different meanings associated with the term "black steel." Here are a couple of possibilities:

1. Black Oxide Coating: "Black steel" can refer to regular steel that has undergone a black oxide coating process. Black oxide coating is a surface treatment used to provide steel with improved corrosion resistance and an attractive black finish. The process involves chemically converting the steel's surface to create a layer of black iron oxide. This black coating can provide some additional protection against rust and wear.
2. Carbon Steel with a Darkened Appearance: "Black steel" can also refer to carbon steel that has a darkened or blackened appearance due to oxidation or intentional surface treatment. This can occur through natural oxidation processes over time, resulting in a darkened patina on the steel's surface. Some swordsmiths or artisans may intentionally darken the steel using techniques like heat treatment or chemical treatments to achieve a specific aesthetic effect.

It's important to note that these terms are not standardized, and the use of "black steel" may vary depending on the specific context. However, both the black oxide coating and the darkened appearance of carbon steel are real and can be seen in various applications, including tools, knives, and decorative items.

If you have a specific product or material in mind when referring to "black steel," it would be helpful to provide more details for a more accurate and specific response.