The most common and most likely answer to this crossword clue is the 6 letter word SMUTTY
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More clues leading to the result SMUTTY
Buddy explains!

The straight part
We know that "‘blue’" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ‘blue’ = smutty.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 SMUTTY mean?
SMUTTY verb- To make dirty; to soil.
- Soiled with smut; blackened, dirty.
- Obscene, indecent.
- Affected with the smut fungus.