These tennis players are appearing so blued (7)

I believe the answer is:

doubles

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'these tennis' is the definition.
(doubles is a kind of tennis)

'players are appearing so blued' is the wordplay.
'so blued' can be anagrammed to 'DOUBLES'.
But, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for doubles that I've seen before include "Tennis match for four players" , "Made twice as much" , "Like Carton and Darnay" , "match in court" , "sort of game" .)

I've seen this clue in the Sydney Morning Herald.

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