Final answer:
A negative cross-price elasticity of demand between blueberries and yogurt indicates that they are complementary goods, meaning the demand for one is positively related to the consumption of the other.
Step-by-step explanation:
If the cross-price elasticity of demand between blueberries and yogurt is negative, this implies that these two goods are complements. This is because a negative cross-price elasticity of demand indicates that an increase in the price of one good leads to a decrease in the quantity demanded of the other good. Specifically, if the price of yogurt increases, the demand for blueberries will also decrease, showing that these goods are often used together, and the demand for one is positively related to the consumption of the other.