The most common and most likely answer to this crossword clue is the 4 letter word ANIL
A
N
I
L
More clues leading to the result ANIL
- Shrub, or a flower in Africa, chopped
- Colouring of Gregorian illuminations
- A flower almost deep blue
- Indigo metal block
- Dye brown paper, cutting top and bottom
- Not entirely used up, culinary dye
- A river, not English, dark blue
- A love for indigo
- Colour of a duck?
- What’s replaced woad? It makes some Roman ill-at-ease
Buddy explains!

The straight part
We know that "dark bl/e" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dark bl/e = anil.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.