Blue material turning stomachs (4)
I believe the answer is:
smut
'blue material' is the definition.
(I've seen this before)
'turning stomachs' is the wordplay.
'turning' says the letters should be written backwards.
'stomachs' becomes 'tums' ('tum' can be a synonym of 'stomach').
'tums' backwards is 'SMUT'.
(Other definitions for smut that I've seen before include "Indecent talk or piece of soot" , "Soot flake; fungal disease" , "Obscene material" , "Dirt or obscenity" , "Dirt or immoral talk" .)