Black spot that's faintly blue (4)
I believe the answer is:
smut
'black spot' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for smut that I've seen before include "Soot flake or sexual innuendo" , "Smudge of soot" , "Sooty matter" , "Indecent items or talk" , "Dirty stuff" .)