This is the 2741st most frequent French word.
bleue
The feminine form of "blue."
Here, 'bleue' is an adjective agreeing with 'mer', a feminine singular noun, to describe its color.
La mer est bleue et calme aujourd'hui.
The sea is blue and calm today.
In this sentence, 'bleue' is an adjective modifying 'robe', a feminine singular noun, specifying its color.
Elle porte une robe bleue pour la fête.
She is wearing a blue dress for the party.
'Bleue' indicates the color after preposition 'en', agreeing with the implied feminine plural object of the adjective.