Dark mark or stain (4)
I believe the answer is:
blot
I believe this clue is a double definition.
'dark mark' is the first definition.
(I've seen this in another clue)
'stain' is the second definition.
(synonyms)
(Other definitions for blot that I've seen before include "Ink stain; disgrace" , "Dark mark/stain" , "Blemish made with ink" , "Don't do it to your copybook" , "stain on reputation" .)