Smudge, stain (4)
I believe the answer is:
blot
I believe this is a double definition.
'smudge' is the first definition.
(synonyms)
'stain' is the second definition.
(I know that stain can be written as blot)
(Other definitions for blot that I've seen before include "Ink stain; disgrace" , "Smudge of ink" , "Don't do it to your copybook" , "Spot of ink, paint etc" , "Smudge (on escutcheon?)" .)