It's an interesting idea. Depends on how you intend to implement it. First, for a black market to exist, there has to be items that can't be obtained legally. Just having stolen versions of the same items available legally won't drive for much of a market value. What kind of objects would you consider to fall in that category? Powerful weapons? Drugs? Information?
Second, who do you want to implement it? Players? NPC's? If players do, then there has to be someway for them to obtain or make the illegal items. If NPC's are the focus of it, then the developers have to control that market with items they make available. Perhaps they could make some very good items show up randomly on a black market vendor that appears now and then in various parts of the city.