Black stuff that could be blue (4)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'black stuff' is the first definition.
(flake of soot)
'could be blue' is the second definition.
('blue' can mean pornographic or smutty)
'that' is the link.
(Other definitions for smut that I've seen before include "A black mark or indecent talk", "particle", "speck", "Dirty jokes - black mark", "Sooty matter", "Sooty deposit", "Mildly obscene remarks", "Small flake of dirt", "Dirty stuff" and "Dirt or immoral talk".)