The term 'Flue Blackener' refers to a job that involves the application of SOOT, which is a black powdery substance, to the inside of a chimney or flue. This is done to protect the flue from corrosion and to improve its efficiency. The SOOT also helps to reduce the amount of smoke produced when burning fuel. The job of a Flue Blackener requires a great deal of skill and precision in order to ensure that the SOOT is applied evenly and correctly.