The riddle
What Do You Call a Sad Strawberry?
Blue as a mood
Strawberries are red, so the joke hands one a new color to match its mood. Wiktionary lists blue as a color and, informally, as a word for sad or low in spirits, the sense in "feeling blue". A strawberry in that mood is a blue berry, and the fruit with exactly that name already exists. One color word does two jobs, and the listener supplies the second one.
How a blueberry gets its color
Wikipedia's article on blueberries says the fruit is pale greenish at first, then reddish-purple, and uniformly blue only when ripe. It wears a powdery layer of wax known as the bloom. The berries measure 5 to 16 millimeters across (about 3/16 to 5/8 of an inch). The commercial kinds, wild and cultivated, are all native to North America, and the tall cultivated bushes did not reach Europe until the 1930s.
A red stage on the way
There is a small coincidence in the answer. Before a blueberry turns blue it passes through a reddish stage, so the fruit that ends up as the punch line starts out closer to the strawberry's color. It is true of the real berries and makes a pleasant detail to share with a group.
Trying it on a group
Ask the question and wait for guesses. They usually run through other colors, pink or gray, before someone lands on blue. Then have everyone pick a fruit and a mood for a new version. A second question that gives a voice to something small is what did the triangle say to the circle.