Patent and Trademark Office, is titled "Anodized Part Having a Matte Black Appearance," and outlines the characteristics of the finish and possible manufacturing processes to achieve it.
The finish comprises an anodized layer that includes "randomly distributed light-absorbing features that are capable of absorbing visible light." The layer contains pores, "where color particles are infused within the pores." The resulting surface is a deep, intense matte black.The patent explains that "merely depositing dye particles within pores of an anodized layer is insufficient to impart a true black color.".
In etching the surface of an anodized layer with pores, Apple is able to absorb "generally all visible light" to deliver a truer black finish without increasing glossiness.