Dark stain (4)
I believe the answer is:
blot
'dark stain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for blot that I've seen before include "Stain left by spilled ink" , "Spot of ink, paint etc" , "bad mark" , "Dry piece of writing" , "Stain (on one's character?)" .)