The Black Tortoise is one of the Four Symbols of the Chinese constellations. It is usually depicted as a tortoise intertwined with a snake. The character '武' can mean 'martial' or 'warrior.' The two characters 玄 and 武 do not have any literal meaning of tortoise and snake. But both tortoise and snake are known to hibernate during winter. The image of intertwined tortoise and snake likely symbolizes a state of inner struggle or a state of hibernation, and thus implies the season of winter. So the English translation Black ~ Dark ~ Mysterious Warrior is a more faithful translation.
It represents the north and the winter season, thus it is sometimes called Black Warrior of the North (北方玄武; Běifāng Xuánwǔ).
In Japan, the characters 玄武 are pronounced as Genbu. It is said to protect Kyoto on the north side, being one of the four guardian spirits that protect the city. It is represented by the Kenkun Shrine, which is located on top of Mount Funaoka in Kyoto.
An important Taoist priest also has 玄武 Xuanwu as his clergy name. He is sometimes (as in Journey to the West) portrayed in the company of a turtle and a snake.