Answer: sootier

AnswerCrossword Clue
SOOTIER

Blacker

SOOTIER

Like a busier sweep?

SOOTIERLike a busier sweep
SOOTIER

More blackened.

SOOTIER
More fuliginous
SOOTIER
More blackened
SOOTIERLike Santa after returning to the North Pole?
SOOTIERMore dirty, as Santa's boots
SOOTIERHaving swept more chimneys?
SOOTIERMore carbon-covered