I came up with this after hearing about the black market card but before getting the actual rules for it.
I think it would work with or without the black market card in play.
Make a black market stack using the place holder cards not in the game (or any variation you like).
Place the stack shuffled, face down next to the supply.
You may use one (action/buy/card draw/$2) to reveal the top black market card to any # of players & replace it.
Using your buy you may name a card & pay it's cost (minimum $2) if it is the top black market card gain a copy of that card. Then the black market stack is re-shuffled.
There are so many ways to vary the black market, Here's just a couple:
Illegal activity- Add curse to black market & if it's the top card immediately gain it if you reveal it.
It fell off a truck- use buy + pay $3 gain a random black market card.
If you use any cards already in the supply you gain the card for one less $.
Use multiple (actions/buys/card draws/$2) to reveal multiple black market cards.
The top card doesn't change until IT is bought.