Black spot (4)
I believe the answer is:
smut
I believe this clue is a double definition.
'black' is the first definition.
(thesaurus)
'spot' is the second definition.
(I have seen 'black spot' mean 'smut' so perhaps 'spot' could also mean 'smut')
(Other definitions for smut that I've seen before include "Flake of soot or dirt" , "Smudge of soot" , "Dirt or obscenity" , "Obscene word(s)" , "Obscene filth" .)