What do you call a sad cranberry?

This joke is a play on words involving the names of two different types of berries and the concept of emotions. Normally, “blue” is a term used to describe a feeling of sadness or depression. The joke takes the word “blueberry,” which is actually the name of a fruit, and uses it to answer the question about a “sad cranberry,” suggesting that if a cranberry were sad, it would be “blue,” just like the name of the fruit “blueberry.” The humor comes from the unexpected connection between the emotion implied by the color “blue” and the actual name of a different berry. It’s a simple pun that plays on the double meaning of the word “blue.”