Black dragons are not actually black, but rather more dark brown but they do possess extraordinary abilities. One such ability is invulnerability (Their scales are harder than any armor).
In the Wiedźmin: Gra Wyobraźni (The Witcher: A Game of Imagination) role-playing game, black dragons are so called because of their dark brown scales. They live mainly in swamps and wetlands, basking the entire day in the mud. They spit streams of acid capable of destroying any armor. Their average length is about 10 meters.
Other work of Andrzej Sapkowski Edit
Black dragon is featured in Sapkowski's The Eye of Yrrhedes role-playing game. It lives in swamps and spit substance which causes the same effect as mustard gas.