Anything that's fuliginous is dirty and blackened, as if it had been rolled in soot. If you're thinking of the fuliginous chimney sweeps in Mary Poppins, you've got the right idea.
The Latin root of fuliginous is fuligo, or "soot," and the word became popular in 16th-century London, when the sooty ash from coal fires could be found everywhere, including in the air itself. From the fuliginous sky in a smoggy town to the fuliginous cat playing in the fireplace, the word also came to figuratively mean "dark or obscure." You can even describe someone's dark sense of humor, pessimistic outlook on life, or bleak mood as fuliginous.