Magicians wear black to stand out from everybody else. The color is mysterious and powerful, which makes it a good choice for a magician's clothing. Black tuxedos are usually worn by magicians for big stage shows, to make them look more powerful and mysterious.
Robes shield from the sun, and dry fast after a rain. Robes, like the turtle's shell, denote that no matter where he may wander, the wizard is at home.