It doesn't "block" anything. It makes things riskier. Kind of like Patrolling The Streets for infiltrators. If you limit Black Market deals to your current location only, then there could be a warning such as "You notice there are quite a few guards monitoring trades. Illegal activities would probably be much riskier right now."
Oh I misunderstood. Thought it would be 100%. Yeah I that sounds much more interesting and it will also give bankers something to do as well.
