Mostly blue flower, one hears (5)

I believe the answer is:

bloom

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

'mostly blue flower' is the definition.
'bloom' can be an answer for 'flower' (synonyms). I am unsure of the 'mostly blue' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for bloom that I've seen before include "(Come into) flower" , "look good" , "Do very well" , "Orlando" , "Flush" .)

I've seen this clue in the Evening Standard.

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