Black stuff that could be blue (4)
I believe the answer is:
smut
I believe this is a triple definition!
'black' is the first definition.
(thesaurus)
'stuff' is the second definition.
(I have seen 'Obscene stuff' mean 'smut' so perhaps 'stuff' could also mean 'smut')
'that' acts as a link.
'could be blue' is the third definition.
('blue' can mean pornographic or smutty)
(Other definitions for smut that I've seen before include "Soot flake; cereal disease" , "Flake of soot or dirt" , "Salacious material" , "Soot, dirt, grit" , "Small flake of dirt" .)