The size of your pieces will effect the color of your filling. If you finely chop (or grate) your chocolate it will have a much darker appearance. If you leave your pieces slightly larger, around 5 millimeters or 1/4 inch and below, your pie will have a freckled appearance similar to the pie which I have made.
1 ounce of unsweetened chocolate is generally measured into 2 blocks in packages. In any case, consult the packaging the be sure.