A Butterfly tastes with her feet. Human finger nails grow faster than their toe nails, and of Human finger nails the middle finger of the dominant hand grows faster than all other Human finger nails. It's these bits of random information that interrupt progress when a person is trying to find out what a "Maypop" might be.
The long answer is rather tedious, involving as it does my own reaction to any thing described as 'tasting like Blackberry,' as well an opinion or two on the the Goji Berry, along with the allergic reaction to the words "health food." The short answer is much more straight forward. A "Maypop" is a Passion Fruit.