A more thorough explanation:

Definition: Blood border is a slang term used to describe the boundary between two states that have different minimum drinking ages. It refers to the practice of young people driving across the border to buy and consume alcohol in the state with a lower minimum age, and then driving back home while intoxicated.

• When I was in college, we used to drive to Indiana from Illinois to buy beer because the drinking age was 18 there. We called it crossing the blood border.
• Some teenagers in New York drive to Pennsylvania to buy alcohol because the minimum drinking age is 21 in New York and 18 in Pennsylvania.

These examples illustrate how young people often take advantage of the different drinking laws in neighboring states. They cross the blood border to purchase alcohol legally and then drive back home while under the influence, which is dangerous and illegal.