The Black Market is a feature in Combat Arms EU. It is a place similar to the Shop, except the weapons bought there are only re-skins of default ones, or ones with slightly different stats. The Black Market also uses NX instead of GP as its currency. NX is earned by buying and activating Wallie cards (Nexon Gamecards in the USA).
In the Market, NX is used to buy things such as Mercenaries, and Special items, like Clan Emblems, Clan Name Changes, and special Uniforms.