A black velvet is made by filling a tall flute glass halfway full of chilled Guinness, and floating the sparkling wine on top of the Guinness; the differing densities of the liquids causes them to remain in separate layers.
Whatever the top layer is, the effect is best achieved if it is poured over a spoon turned upside down over the top of the glass, so it runs down the sides rather than mixing.
A similar effect is achieved by mixing Guinness and a light-colored beer in the black and tan cocktail.
According to Brewer's Dictionary of Phrase and Fable, the Black Velvet was the favorite drink of Otto von Bismarck.
See also: list of cocktails