I have answered this question on Quora:
What’s a good symbol or image for poetry other than the raven?
A Blue Flower was a central symbol of inspiration for the Romanticism movement, and remains an enduring motif in Western art today. It stands for desire, love, and the metaphysical striving for the infinite and unreachable. It symbolizes hope and the beauty of things.
German author Novalis used the symbol in his unfinished Bildungsroman, entitled Heinrich von Ofterdingen. After contemplating a meeting with a stranger, the young Heinrich von Ofterdingen dreams about a blue flower which calls to him and absorbs his attention.
The blue flower symbolizes the joining of humanity with nature and the spirit so the understanding of nature and of the self is growing.
Art – Vincent van Gogh