This is the 1464th most frequent French word.
bleu
Blue.
The word 'bleu' is used as an adjective describing the noun 'ciel' (sky), providing information about its color.
Le ciel est bleu aujourd'hui.
The sky is blue today.
Here 'bleu' is also an adjective describing the noun 'couleur,' specifying the preference in terms of color.
Elle préfère les chemises de couleur bleu.
She prefers shirts of blue color.
In this sentence, 'bleu' functions as an adjective describing 'yeux' (eyes), comparing their color to that of the ocean.