When lens barrels were made of brass, chrome plating was the easiest way to finish them. Chrome plates readily onto brass and its hardness protects the softer brass. Once aluminum became the material of choice for lens barrels, chrome plating was no longer needed or practical. Chrome plating aluminum is difficult and problematic. Anodizing is the natural finishing process for aluminum and it can be dyed various colors. To avoid being garish, black was the natural choice. It also matches the rubber grips that were put onto SLR lenses.