The most common and most likely answer to this crossword clue is the 6 letter word SMEARS
I've seen this crossword clue in The Guardian
S
M
E
A
R
S
More clues leading to the result SMEARS
Buddy explains!
The straight part
We know that "Blackens" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case blackens = smears.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SMEARS mean?
SMEARS verb- To spread (a substance, especially one that colours or is dirty) across a surface by rubbing.
- To have a substance smeared on (a surface).
- To damage someone's reputation by slandering, misrepresenting, or otherwise making false accusations about an individual, their statements, or their actions.
- To become spread by smearing.
- To climb without using footholds, using the friction from the shoe to stay on the wall.