E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A corps of 700 volunteer hussars under the command of Frederick William, Duke of Brunswick, who had been forbidden by Napoleon to succeed to his fathers dukedom. They were called Black because they wore mourning for the deceased Duke. Frederick William fell at Quatre-Bras, 1815. One of Millaiss best pictures is called The Black Brunswicker.