The answer to the riddle, "What is the saddest fruit?" is "blueberry," and it cleverly plays on the dual meaning of the word "blue." In one sense, "blue" can describe the color of the fruit, while in another, it is a term often associated with feelings of sadness or melancholy. Thus, the riddle humorously combines the literal color of the fruit with its implied emotional state, making "blueberry" the perfect answer!