Fast Black K Salt is primarily used in the textile industry as a diazo component in the dyeing process. It is particularly effective for producing deep black and dark shades on cotton, silk, and rayon fabrics. The chemical is applied in combination with coupling agents to form stable, colorfast dyes that are resistant to washing and light exposure. Additionally, it is utilized in the paper industry for coloring papers and in the leather industry for dyeing leather products. Its ability to produce intense and durable colors makes it a preferred choice for applications requiring long-lasting and vibrant finishes.