Randy Blye of Birmingham, Alabama supplies some technical information on the characteristics of Mirror Black.
The interior color of Carder’s Mirror Black is as Erickson suggested potassium permanganate or a shade of purplish red. Most blacks have an interior color of cobalt blue. If you take the piece outside and hold it directly to the ball of the sun you can usually see a small dot of the color.

There is a suggestion that the black colored with cobalt made a black that was too hard for cutting or etching. It is my opinion that Carder’s chemical composition of black provided more of a mirror reflection to it’s surface. Perhaps both were a factor in his choice.