Steel blue is a shade of blue similar to steel blue, i.e. blue. h Blackened steel to protect against rust. It is one of the least vibrant shades of blue and is commonly known as the blue-gray color. The first recorded use of Steel Blue as a color name in English was in 1817. In 1987, Steel Blue was listed as one of the X11 colors, later also known as X11 Web Colors after the invention of the World Wide Web in 1991. .