He was named after his birthplace. The sobriquet ‘Black Prince’ does not appear in written records until the 16th century, nearly two centuries after his death, though as a nickname it may date back to his lifetime.
The origin of the term is just as obscure as the date it was first used. It may stem from Edward’s habit, when jousting, of putting aside his royal coat of arms in favour of a black ‘shield for peace’ decorated with three white ostrich feathers.
Some historians believe he also wore black armour, while others have suggested that the name may have been derived from the French habit of referring to a particularly brutal commander as a ‘black boar’.
In truth, we do not know for certain. Via The excellent ‘History Revealed’ Website