Variation in same key will make it really dark (3,4)

I believe the answer is:

eye mask

'will make it really dark' is the definition.
(someone wearing an eye mask is in the dark)

'variation in same key' is the wordplay.
'variation in' indicates an anagram.
'same'+'key'='samekey'
'samekey' is an anagram of 'EYE MASK'.

I've seen this clue in The Guardian.

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