Black Market Affiliation gets you cheaper bullets and knives through a black market contact. It starts the Swindler discount line.
Black Market Affiliation is entry 3 of 13 in the Swindler branch, one of the three branches (Ringleader, Insider and Swindler) on the Rooks' 39-upgrade board. It is bought with money and materials, not skill points: £3,000, 50 Metal, 50 Chemicals. Before Black Market Affiliation unlocks you need Medicine Market.