When sourcing black core paper, the primary indicator of quality is opacity and light-blocking capability. You should look for a multi-layer structure where a specialized black opaque layer is sandwiched between two outer paper layers. Key specs include Grammage (GSM), typically ranging from 250gsm to 350gsm for professional playing cards, and stiffness/resilience, ensuring the card returns to its original shape after bending. Additionally, verify the surface coating (such as air-cushion finish or UV oil) which affects the 'slip' and durability during shuffling.