Premium Black cards are produced on black card stock. This is unique as most cards are white (and printed with black ink to appear black); the edges are black, the inside of the card is black (if you tear it apart).
As the card is black, printed ink will not be visible over the dark paper. Design for Premium Black Cards cannot be produced with ink and will need to be done with a foil stamping or screen printing effect (see: What foil colours can I select from?). The cards can also be embossed or 3D embossed to raise parts of the card.
These cards are a premium product due to the high quality and calibre of card that is used – as well as the higher production costs from foiling / embossing services (rather than printing).