Great question! Black markets are a subject which seem to come up in every freshman course.
To show what happens when a good is made illegal, we first need to illustrate what the supply and demand for the good looked like in the pre- black market days. We can do so by arbitrarily drawing a downward sloping demand curve (blue) and an upward sloping supply curve (red), as shown on this graph. Note that price is on the X-axis and quantity is on the Y-axis.
On the next page, we will show what happens when the government makes the product illegal, thus creating a black market.