Like Young witch, it really depends of the added card : strong if it is mountebank, weak if it is explorer.
If the added "forbidden" card is an $5 action, and what about gaining the card. Can you remodel a silver into a <forbidden card> or gain it from university ?
And talking about mountebank as the added card (also works with witch and somewhat ill-gotten gains), it leads a bit to the same problem as familliar : you need $3 plus a specific card to buy it (EDIT : when the forbidden card costs $5, so in most of the cases), so the probability of getting it or not is the same, and there are already a graat luck factor on getting $3P on turns 3-4 when familiars...
Otherwise, if it is a good non-cursing card (h.party, festival, goons), I'm afraid it could lead to kinda slow games.
But it's still a better and more interesting idea than this damn black market. The concept is good !!!