Down and dirty (4)

I believe the answer is:

blue

This clue is a double definition.

'down' is the first definition.
(both can mean sad)

'dirty' is the second definition.
(blue can mean pornographic)

'and' is the link.

(Other definitions for blue that I've seen before include "in Oxford, it's dark" , "Unsuitable for young" , "Sad; butterfly" , "adult" , "very cold" .)

I've seen this clue in The Sun, The Independent, The Telegraph and The Guardian.

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