Basically cards graded by Beckett are also given subgrades from 1-10 for Centering/Surface/Edges/Corners as well and if a card achieves a 10 in all subcategories it gets a black label instead of the standard gold ones. Refer to:
www.elitefourum.com/t/the-first-only-bgs-black-label-pristine-pokemon-card/12639/1
Although other black label Pokemon cards exist now, I’m fairly certain it’s still in the single digits.