In Victorian times it was considered socially disastrous to wear shiny faceted jewels or shiny metal during the year-long period of deep mourning as it was a sign of coming out of mourning too early. Matte black metal mourning straight pins such as these were used to hold veils and other clothing accessories in place during this period. Bright finishes were allowed to be used in the later stages of mourning, or half-mourning.
Mirrors were sometimes covered, and I’ve heard that cloth was sometimes hung up in front of photos of the deceased. Perhaps mourning pins were used to hold these cloths as well.