Crossword Genius

When blue, it flies (6)

Ross

I believe the answer is:

bottle

I'm a little stuck... Click here to teach me more about this clue!

This could be a double definition.

'when blue' is the first definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?

'it flies' is the second definition.
The definition suggests a singular noun which matches the answer.

Can you help me to learn more?

(Other definitions for bottle that I've seen before include "confidence" , "Narrow-necked vessel" , "Glass or plastic container" , "Grit" , "beer found in this?" .)

I've seen this clue in the Evening Standard.
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