être fleur bleue

to be hopelessly romantic and sentimental

This expression describes someone who is very romantic, dreamy, and sentimental.

Ne te moque pas de moi, je suis un peu fleur bleue et j'adore les comédies romantiques.

Don't make fun of me, I'm a bit of a hopeless romantic and I love romantic comedies.

Comes from the German Romantic novel 'Heinrich von Ofterdingen' (1802) by Novalis, in which the hero dreams of a mysterious blue flower that symbolizes romantic longing and the unattainable ideal. The symbol spread across European Romanticism and entered French as this expression.