The most common and most likely answer to this crossword clue is the 9 letter word LAMPBLACK
I've seen this crossword clue in The Guardian
L
A
M
P
B
L
A
C
K
Buddy explains!
The straight part
We know that "dark pigment" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dark pigment = lampblack.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 LAMPBLACK mean?
LAMPBLACK noun- An amorphous form of carbon made from incompletely burned organic matter; used to make pigments and inks.
- To blacken using this form of carbon.