- A sharp drop in supply, as the penalties for selling the good causepeople to shift into other industries
- A drop in demand, as a prohibition of posessing the good deters some people from wanting to buy it
The shift in supply and demand causes the quantity consumed of the black market good to decrease, while the price rises. If the demand side effects dominate, we will still see a drop in quantity consumed, but we will also see a corresponding drop in price. This does not typically happen in a black market - we normally see a rise in price.
The amount of the price change and the change in quantity consumed will depend on the magnitude of the shifts of the curve, as well as the price elasticity of demand and the price elasticity of supply.